Methyl Orange-Doped Polypyrrole Promoting Growth of ZIF-8 on Cellulose Fiber with Tunable Tribopolarity for Triboelectric Nanogenerator

Cellulose fiber (CelF) is a biodegradable and renewable material with excellent performance but negligible triboelectric polarizability. Methods to enhance and rationally tune the triboelectric properties of CelF are needed to further its application for energy harvesting. In this work, methyl-orange-doped polypyrrole (MO-PPy) was in situ coated on CelF as a mediating layer to promote the growth of metal–organic framework ZIF-8 and to construct a cellulose-based triboelectric nanogenerator (TENG). The results showed that a small amount of MO-PPy generated in situ significantly promoted the growth of ZIF-8 on CelF, and the ZIF-8 deposition ratio was able to increase from 7.8% (ZIF-8/CelF) to 31.8% (ZIF-8/MO-PPy@CelF). ZIF-8/MO-PPy@CelF remained electrically conductive and became triboelectrically positive, and the triboelectricity’s positivity was improved with the increase in the ZIF-8 deposition ratio. The cellulose-based TENG constructed with ZIF-8/MO-PPy@CelF (31.8% ZIF-8 deposition ratio) and polytetrafluoroethylene (PTFE) could generate a transfer charge of 47.4 nC, open-circuit voltage of 129 V and short-circuit current of 6.8 μA—about 4 times higher than those of ZIF-8/CelF (7.8% ZIF-8 deposition ratio)—and had excellent cycling stability (open-circuit voltage remained almost constant after 10,000 cycles). MO-PPy not only greatly facilitated the growth of ZIF-8 on CelF, but also acted as an electrode active phase for TENG. The novel TENG based on ZIF-8/MO-PPy@CelF composite has cheerful prospects in many applications, such as self-powered supercapacitors, sensors and monitors, smart pianos, ping-pong tables, floor mats, etc.


Introduction
In the past few decades, due to the significant increase in energy consumption, people have paid more attention to renewable energy. Finding renewable energy to reduce carbon emissions, ensure long-term energy supply and reduce dependence on fossil fuels is the inevitable requirement of sustainable economic development. Various methods have been proven to be able to convert renewable energy [1][2][3][4][5][6][7][8][9]. Among energy harvesting technologies, the triboelectric nanogenerator (TENG), based on the combination of triboelectrification and electrostatic induction, was fabricated for the efficient conversion of various forms of green and renewable energy into electrical energy [10][11][12][13][14]. TENG operates based on the contact of two different friction materials with opposing tendencies of electron affinity. More precisely, triboelectrification supplies opposite polarization charges on the surface of each contact material, and mechanical energy is then converted into electrical energy driven by electrostatic induction [15]. In addition to this facile operation mode, TENG has many advantages such as light weight, high cost-benefit, facile manufacturing and high output voltage at low frequencies [16,17]. Thus, TENG is a promising technology in the field of energy harvesting.

Preparation of MO-PPy@CelF Composite
MO-PPy@CelF composite was prepared by in situ polymerization process. First, 2 g of oven-dry CelF was placed in 150 mL distilled water, and 0.1 g of MO was added. Then, 1 mL of Py was added to the above mixture. Next, 4.1 g of FeCl3 · 6H2O, dissolved in 50 mL of distilled water, was slowly added to the mixture and stirred continuously at 0-5 °C for 2 h. Finally, MO-PPy@CelF composite was obtained by washing with distilled water. For comparison, a PPy@CelF composite without the addition of MO was also prepared.

Preparation of ZIF-8@CelF Composite
ZIF-8@CelF composite was synthesized by simple in situ growth method. First, 2 g of CelF (oven-dry) was added to 150 mL of mixed solvent (the volume ratio of water and Scheme 1. Schematic illustration of preparation and application of ZIF-8/MO-PPy@CelF composite. With the exception of Py monomer, which was distilled under reduced pressure before use, all chemicals were directly used without further purification.

Preparation of MO-PPy@CelF Composite
MO-PPy@CelF composite was prepared by in situ polymerization process. First, 2 g of oven-dry CelF was placed in 150 mL distilled water, and 0.1 g of MO was added. Then, 1 mL of Py was added to the above mixture. Next, 4.1 g of FeCl 3 · 6H 2 O, dissolved in 50 mL of distilled water, was slowly added to the mixture and stirred continuously at 0-5 • C for 2 h. Finally, MO-PPy@CelF composite was obtained by washing with distilled water. For comparison, a PPy@CelF composite without the addition of MO was also prepared.

Preparation of ZIF-8@CelF Composite
ZIF-8@CelF composite was synthesized by simple in situ growth method. First, 2 g of CelF (oven-dry) was added to 150 mL of mixed solvent (the volume ratio of water and methanol is 1:1) containing a certain amount of Zn(NO 3 ) 2 · 6 H 2 O for 3 h. Then, 2-MI (the Polymers 2022, 14, 332 4 of 17 molar ratio of 2-MI to zinc nitrate is 4:1), dissolved in 50 mL of mixed solvent, was added and stirred magnetically. Finally, ZIF-8@CelF composite was obtained by washing and drying at 60 • C.

Preparation of ZIF-8/MO-PPy@CelF Composite
ZIF-8/MO-PPy@CelF composite was prepared by double in situ method. First, 2 g of CelF (oven-dry) and 0.1 g of MO were added to 150 mL distilled water. Then, 1 mL of Py was added to the above mixture. 4.1 g of FeCl 3 · 6 H 2 O, dissolved in 50 mL of distilled water, was slowly added to the mixture and stirred continuously at 0-5 • C for 2 h. MO-PPy@CelF composite was obtained by washing with distilled water. Next, 2 g of MO-PPy@CelF (oven-dry) was added to 150 mL of mixed solvent (the volume ratio of water to methanol is 1:1) containing a certain amount of Zn(NO 3 ) 2 · 6 H 2 O and stirred for 3 h. Then, 2-MI (the molar ratio of 2-MI to zinc nitrate is 4:1), dissolved in 50 mL of mixed solvent, was added and stirred magnetically. ZIF-8/MO-PPy@CelF was obtained by washing with distilled water. The uniformly dispersed ZIF-8/MO-PPy@CelF suspension was poured into a sand core funnel fitted with filter paper. After vacuum filtration, the wet ZIF-8/MO-PPy@CelF paper sheet was removed and dried at 105 • C for 16 min (8 min each side). The thickness of the ZIF-8/MO-PPy@CelF composite paper was measured using an IMT-HD02 thickness tester produced by Dongguan Gaoxin Testing Equipment Co., Ltd. (Dongguan, China).

Calculation of ZIF-8 Deposition Ratio
The as-synthesized simple was dried at 105 • C for 3 h. After cooling for 30 min in a dryer, the mass was measured. The deposition ratio (D, %) of ZIF-8 was calculated using the following formula: where M 0 is the original mass of CelF, g; M 1 is the mass of the composite coated MO-PPy, g; M 2 is the mass of the composite deposited ZIF-8, g.

Fabrication of C-TENG
Polytetrafluoroethylene (PTFE) film (4 × 4 cm 2 ) was used as the negative friction layer. The ZIF-8/MO-PPy@CelF (4 × 4 cm 2 ) was used as the positive friction layer and electrode material. MO-PPy@CelF was used as a bottom electrode, attached on bottom of a PTFE film. Polymethyl methacrylate (PMMA) boards were cut and used as support materials for the TENG. Top and bottom parts were attached to two PMMA boards. The positive and negative copper wires were connected to the friction material and the electrode, respectively. Positive and negative copper wires were then connected to a Keithley 6514 electrometer produced by Keithley Instruments, Inc. (Cleveland, OH, USA) to test the output performance of the TENG. A JZK-10 modal exciter produced by Jiangsu Lianneng Electronic Technology Co., Ltd. (Yangzhou, China) was used as an external force source. In this work, the force exerted by the contact separation was 50 N, and the frequency during the experiment was 2 Hz.

Characterization
The morphology of samples was analyzed using a Zeiss sigma 300 scanning electron microscope (Carl Zeiss AG, Oberkochen, Germany) with intelligent EDX spectrometer at an accelerating voltage of 0.02-30 kV. Before observation, the sample surface was coated with gold under vacuum. The wavelength of the nickel-filtered Cu-Kα radiation source was 1.5418 Å, the voltage was 40 kV and the current was 40 mA. Thermo Scientific Nicolet iS5 FTIR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) was used to record the FTIR spectra of samples in the frequency range of 4000-400 cm −1 . The element composition and valence information of samples was measured using ESCALAB 250xi X-ray photoelectron spectrometer (Thermo Fisher Scientific, MA, USA) with Al-Kα radiation source (hv = 1486.6 eV). RIGAKU Ultima IV X-ray diffractometer (Beijing Guanyuan Technology Co., Ltd., Beijing, China), with a scanning range of 5-90 • and scanning speed of 5 • /min, was used to analyze the crystalline nature of the samples. The Kelvin probe force microscopy (KPFM) measurement was taken on an AFM system using Pt/Ir-coated SCM-PIT probe (Bruker Corporation, Billerica, MA, USA). RTS-8 four probes tester (Guangzhou Four Probe Technology Co., Ltd., Guangzhou, China) was used to determine the conductivity of the samples.

Promoting Effect of MO-PPy on the Growth of ZIF-8
As shown in Figure 1, the ZIF-8 deposition ratio on CelF was effectively increased up to 31.8% when MO-PPy was used as the mediating layer. In contrast, without PPy, the deposition ratio of ZIF-8 on CelF was only 7.8%. When using PPy without MO doping, the deposition ratio of ZIF-8 was 14.4%, indicating that the MO-PPy in situ coated as a mediating layer can significantly promote the growth of ZIF-8 on CelF. The promoting effect of MO-PPy on the growth of ZIF-8 on CelF was mainly attributed to the adsorption of metal ions by NH groups of PPy [47] and azo groups of MO molecules [48]. The effects of process variables such as temperature, time, Py and zinc nitrate dosages on the deposition ratio of ZIF-8 on CelF were also investigated. As shown in Figure 2a, time had a noticeable effect on the deposition ratio of ZIF-8. The deposition ratio of ZIF-8 increased rapidly in the first hour, after which it increased slowly. The deposition ratio of ZIF-8 reached 31.8% at 6 h. At the initial stage, the amount of ZIF-8 deposited was low for a short period of time as the added metal ions had not yet fully reacted with the ligand, which was attributed to the low amount of nucleation of ZIF-8 at this time. The deposition ratio of ZIF-8 increased significantly with the prolongation of time, which was attributed to the full reaction of the metal ions with the ligand. Temperature had almost no influence on the deposition of ZIF-8 on CelF (Figure 2b), indicating that ZIF-8 in situ growth has an extremely low activation energy. As shown in Figure 2c, ZIF-8 deposition ratio decreased with Py dosage; its value was the highest when Py dosage was 1 mL, indicating that a small amount of MO-PPy as a mediating layer can promote the growth of ZIF-8 on CelF, and the thick MO-PPy coating is not conducive to the deposition of ZIF-8. The deposition ratio of ZIF-8 increased with increasing zinc nitrate dosage, reaching a deposition ratio of 21.2% when zinc nitrate dosage was 12 mmol (Figure 2d).

FTIR Analysis
The FTIR spectra of CelF, MO-PPy@CelF, ZIF-8 and ZIF-8/MO-PPy@CelF were recorded; the results are displayed in Figure 5. The spectrum of CelF had obvious characteristic peaks of cellulose. The wide peak at about 3300 cm −1 was attributed to the O-H stretching vibration, and the peak at about 2990 cm −1 was ascribed to the C-H stretching vibration. After the deposition of MO-PPy, the O-H stretching vibration peak at 3300 cm −1 and the C-H stretching vibration at 2990 cm −1 were significantly weakened, which indicates that the cellulose was completely covered. The absorption peak at about 1630 cm −1 in the spectrum of MO-PPy@CelF was assigned to the N-H stretching vibration of PPy, demonstrating the successful decoration of PPy on pure CelF. For ZIF-8, the peak at 1584 cm −1 corresponded to the C=N stretching of the imidazole ring [50,51]; the convoluted peaks at 1500-600 cm −1 were attributed to the entire imidazole ring stretching or bending effects [52]. It can be found that the FTIR spectrum of ZIF-8/MO-PPy@CelF contained the characteristic peaks of CelF and ZIF-8. The characteristic peaks at 1139, 1310 and 757 cm −1 in the ZIF-8 appeared in ZIF-8/MO-PPy@CelF, indicating the successful growth of ZIF-8 nanoparticles on MO-PPy@CelF.

XPS Analysis
The XPS spectra of CelF, MO-PPy@CelF, ZIF-8 and ZIF-8/MO-PPy@CelF further provided rich information about the chemical state of the surface elements of the composites. As shown in Figure 6a, C and O peaks appeared in all samples, which were attributed to the rich hydroxyl groups in the carbon skeleton of organic compounds and cellulose matrix. The N element peak represents MO-PPy and ZIF-8, and the Zn element peak also represents ZIF-8. The above characteristic peaks appeared in ZIF-8/MO-PPy@CelF, which indicated that both ZIF-8 and MO-PPy were deposited on the surface of CelF, and ZIF-8/MO-PPy@CelF composite was successfully prepared. Figure 6b shows N 1s narrow scan XPS spectrum of MO-PPy@CelF, and the N 1s could be deconvoluted into three peaks of 399.4, 401 and 402.3 eV, corresponding to -NH-, -NH + -and -N= bonds, respectively [53]. In the XPS spectrum of ZIF-8, the N 1s can be fitted into two peaks at 401.1 eV and 398.4 eV (Figure 6c), which corresponds to pyrrolic N and pyridinic N, respectively [54]. In the XPS spectrum of ZIF-8/MO-PPy@CelF composite, the N1s was deconvoluted into three peaks (Figure 6d). Two peaks at 398.2 eV and 398.8 eV were associated with the -NH-and -NH + -groups of pyrrole unit [55]. The peak at 398.6 eV was attributed to C=N defects of MO-PPy and the coordination of N-Zn [56].

XPS Analysis
The XPS spectra of CelF, MO-PPy@CelF, ZIF-8 and ZIF-8/MO-PPy@CelF further provided rich information about the chemical state of the surface elements of the composites. As shown in Figure 6a, C and O peaks appeared in all samples, which were attributed to the rich hydroxyl groups in the carbon skeleton of organic compounds and cellulose matrix. The N element peak represents MO-PPy and ZIF-8, and the Zn element peak also represents ZIF-8. The above characteristic peaks appeared in ZIF-8/MO-PPy@CelF, which indicated that both ZIF-8 and MO-PPy were deposited on the surface of CelF, and ZIF-8/MO-PPy@CelF composite was successfully prepared. Figure 6b shows N 1s narrow scan XPS spectrum of MO-PPy@CelF, and the N 1s could be deconvoluted into three peaks of 399.4, 401 and 402.3 eV, corresponding to -NH-, -NH + -and -N= bonds, respectively [53]. In the XPS spectrum of ZIF-8, the N 1s can be fitted into two peaks at 401.1 eV and 398.4 eV (Figure 6c), which corresponds to pyrrolic N and pyridinic N, respectively [54]. In the XPS spectrum of ZIF-8/MO-PPy@CelF composite, the N1s was deconvoluted into three peaks (Figure 6d). Two peaks at 398.2 eV and 398.8 eV were associated with the -NH-and -NH + -groups of pyrrole unit [55]. The peak at 398.6 eV was attributed to C=N defects of MO-PPy and the coordination of N-Zn [56].

Working Principle
In a contact-separation operating mode, the working principle of the C-TENG is based on the coupling effect of contact electrification and electrostatic induction [57], as shown in Figure 7. In this work, ZIF-8/MO-PPy@CelF was used as a positive friction layer and electrode material, mainly based on the fact that ZIF-8/MO-PPy@CelF exhibits electrical conductivity ( Figure 8). Meanwhile, MO-PPy@CelF with a conductivity of 9.8 S·m −1 was used as a bottom electrode, attached on the bottom of the PTFE film. In the initial state, no charges generate on the surface of ZIF-8/MO-PPy@CelF composite or PTFE (Figure 7i). Nevertheless, when the two friction layers are in full contact, static electrostatic charges are generated on the two friction materials driven by external forces (Figure 7ii

Output Performance
The surface potential of friction materials plays a critical role in the output performance of the TENG [36]. Here, the surface potential of ZIF-8/MO-PPy@CelF was studied using KPFM, which proved the positive surface potential of ZIF-8/MO-PPy@CelF ( Figure.  9a,b). Specifically, a positive contact potential difference, VCPD, means that electrons can tunnel easily from the surface, i.e., the surface can be positively charged easily when in contact with the other material [36]. Therefore, ZIF-8 can be used as a positive triboelectric material in the TENG. Based on the good performance of ZIF-8, PTFE and ZIF-8/MO-PPy@CelF were selected as the negative and positive friction layers for the fabrication of TENG. The positively charged nature of ZIF-8 may also be related to its internal functional groups. The ZIF-8 structure contains imidazole ligands with -NH functional groups, which tend to contribute electrons, thus making the material containing the -NH functional groups positively charged [58]. Meanwhile, the surface roughness of the friction material also plays an important role in the output performance of TENG [59]. The increase in surface roughness improves the TENG output as it brings more area into contact during the working of the TENG. Figure 9c,d shows the surface roughness values of two ZIF-8/MO-PPy@CelF samples with different ZIF-8 deposition ratios. The ZIF-8/MO-PPy@CelF with 31.8% ZIF-8 deposition ratio is rougher than that with 17.4% ZIF-8 deposition ratio. Therefore, the increase in ZIF-8 deposition ratio is beneficial to the improvement of TENG performance.

Output Performance
The surface potential of friction materials plays a critical role in the output performance of the TENG [36]. Here, the surface potential of ZIF-8/MO-PPy@CelF was studied using KPFM, which proved the positive surface potential of ZIF-8/MO-PPy@CelF (Figure 9a,b). Specifically, a positive contact potential difference, V CPD , means that electrons can tunnel easily from the surface, i.e., the surface can be positively charged easily when in contact with the other material [36]. Therefore, ZIF-8 can be used as a positive triboelectric material in the TENG. Based on the good performance of ZIF-8, PTFE and ZIF-8/MO-PPy@CelF were selected as the negative and positive friction layers for the fabrication of TENG. The positively charged nature of ZIF-8 may also be related to its internal functional groups. The ZIF-8 structure contains imidazole ligands with -NH functional groups, which tend to contribute electrons, thus making the material containing the -NH functional groups positively charged [58]. Meanwhile, the surface roughness of the friction material also plays an important role in the output performance of TENG [59]. The increase in surface roughness improves the TENG output as it brings more area into contact during the working of the TENG. Figure 9c,d shows the surface roughness values of two ZIF-8/MO-PPy@CelF samples with different ZIF-8 deposition ratios. The ZIF-8/MO-PPy@CelF with 31.8% ZIF-8 deposition ratio is rougher than that with 17.4% ZIF-8 deposition ratio. Therefore, the increase in ZIF-8 deposition ratio is beneficial to the improvement of TENG performance. Polymers 2022, 14, x FOR PEER REVIEW 12 of 17 In this work, both the surface potential and roughness of CelF surface can be improved by increasing the deposition ratio of ZIF-8 on CelF surface. Whereas the limited ability of pure CelF to adsorb and chelate metal ions leads to less deposition of ZIF-8 on CelF surface. The MO-PPy generated in situ was used as a mediating layer to increase the deposition ratio of ZIF-8 on CelF surface, thereby improving the performance of C-TENG. Therefore, we focused on the effect of ZIF-8 deposition on the output performance of TENG such as transfer charge, short-circuit current and open-circuit voltage. Figure 10a show an image of the C-TENG. As shown in Figure 10b-d, the transfer charge, open-circuit voltage and short-circuit current of the modified C-TENG increased with an increasing deposition ratio of ZIF-8. When the deposition ratio of ZIF-8 was 31.8%, the transfer charge was the highest (47.4 nC) (Figure 10b). Charge transfer of the triboelectric materials played an important role in the output performance of TENG [60]. The open-circuit voltage increased from 27 V to 129 V (Figure 10c) and the short-circuit current increased from 1.7 μA to 6.8 μA (Figure 10d), approximately four times higher than those of the unmodified C-TENG, as the deposition ratio of ZIF-8 increased from 7.8% to 31.8%. Meanwhile, we calculated the current densities as shown in Figure S2. The good output performance of TENG is mainly attributed to more ZIF-8 deposition on the CelF surface. The more ZIF-8 deposited on the CelF surface, the positive polarity was stronger, and the more surface charge was generated on surface of materials. In this work, both the surface potential and roughness of CelF surface can be improved by increasing the deposition ratio of ZIF-8 on CelF surface. Whereas the limited ability of pure CelF to adsorb and chelate metal ions leads to less deposition of ZIF-8 on CelF surface. The MO-PPy generated in situ was used as a mediating layer to increase the deposition ratio of ZIF-8 on CelF surface, thereby improving the performance of C-TENG. Therefore, we focused on the effect of ZIF-8 deposition on the output performance of TENG such as transfer charge, short-circuit current and open-circuit voltage. Figure 10a show an image of the C-TENG. As shown in Figure 10b-d, the transfer charge, open-circuit voltage and short-circuit current of the modified C-TENG increased with an increasing deposition ratio of ZIF-8. When the deposition ratio of ZIF-8 was 31.8%, the transfer charge was the highest (47.4 nC) (Figure 10b). Charge transfer of the triboelectric materials played an important role in the output performance of TENG [60]. The open-circuit voltage increased from 27 V to 129 V (Figure 10c) and the short-circuit current increased from 1.7 µA to 6.8 µA (Figure 10d), approximately four times higher than those of the unmodified C-TENG, as the deposition ratio of ZIF-8 increased from 7.8% to 31.8%. Meanwhile, we calculated the current densities as shown in Figure S2. The good output performance of TENG is mainly attributed to more ZIF-8 deposition on the CelF surface. The more ZIF-8 deposited on the CelF surface, the positive polarity was stronger, and the more surface charge was generated on surface of materials. As seen in Figure 10e, the voltage increased with increasing external resistance and the output power density increased and then decreased with increasing resistance, reaching a maximum of 33.3 mW·m -2 at a resistance of 30 MΩ. Moreover, as shown in Video S1, the C-TENG was connected to several LED lights in series, and 12 LEDs were successfully lit up with the slap of hands. In addition to excellent performance, the operating stability of the TENG is an important indicator in order to ensure the long-term collection of environmental mechanical energy [61]. As shown in Figure 10f, the open-circuit voltage of the C-TENG remained almost constant after 10,000 cycles at a frequency of 2 Hz and an external force of 50 N, indicating the C-TENG had excellent cycling stability. At the same As seen in Figure 10e, the voltage increased with increasing external resistance and the output power density increased and then decreased with increasing resistance, reaching a maximum of 33.3 mW·m −2 at a resistance of 30 MΩ. Moreover, as shown in Video S1, the C-TENG was connected to several LED lights in series, and 12 LEDs were successfully lit up with the slap of hands. In addition to excellent performance, the operating stability of the TENG is an important indicator in order to ensure the long-term collection of environmental mechanical energy [61]. As shown in Figure 10f, the open-circuit voltage of the C-TENG remained almost constant after 10,000 cycles at a frequency of 2 Hz and an external force of 50 N, indicating the C-TENG had excellent cycling stability. At the same time, due to the hydrophilic nature of cellulose, we found that the output data of TENG decreased at high relative humidity ( Figure S3). This work has made some efforts towards the preparation of positive cellulose-based friction materials. The output performance comparison of C-TENG is shown in Table S1 [27,[62][63][64][65][66][67]. This work successfully deposited ZIF-8 onto CelF using MO-PPy as the mediating layer and the output performance of the assembled C-TENG can be tuned by the deposition ratio of ZIF-8 on CelF. The preparation of ZIF-8/MO-PPy@CelF in this work is simple and green, and the C-TENG can reach an open-circuit voltage of 129 V and a power density of 33.3 mW·m −2 , showing good output performance.
As discussed earlier, the ZIF-8/MO-PPy@CelF-based TENG developed by this work was proven to be the excellent power source for lighting LEDs. Cellulose-based TENGs have been reported to provide power for electronics [68][69][70][71][72]. In this work, TENG based on ZIF-8/MO-PPy@CelF can potentially serve as a power source for supercapacitors to store energy. Flexible sensors based on ZIF-8/MO-PPy@CelF-based TENG are worth developing and are expected to enable tactile sensing and monitoring of various physiological signals such as pulses in the human body. TENG based on ZIF-8/MO-PPy@CelF can also be used in emerging smart necessities such as paper-based energy-harvesting pianos, ping-pong tables, floor mats, etc.

Conclusions
Using MO-PPy as a bridge between ZIF-8 and CelF to enhance the binding, a novel conductive ZIF-8/MO-PPy@CelF composite was successfully fabricated by simple in situ growth strategy. The increase in the deposition ratio of ZIF-8 on CelF from 7.8% (ZIF-8/CelF) to 31.8% (ZIF-8/MO-PPy@CelF) indicates that the MO-PPy generated in situ as a mediating layer is effective in promoting the growth of ZIF-8 on CelF. The factors influencing ZIF-8 deposition are mainly chemical dosages and time; temperature has a negligible effect. The morphology and structure of ZIF-8/MO-PPy@CelF composite were characterized by SEM, XRD, FTIR and XPS. The ZIF-8/MO-PPy@CelF was a positive triboelectric material as confirmed by the KPFM analysis. The surface polarity of CelF significantly increased after ZIF-8 nanoparticles were deposited on CelF, and positive triboelectricity increased with the increase in the deposition ratio of ZIF-8. Using ZIF-8/MO-PPy@CelF and PEFT as friction materials, a C-TENG based on contact-separation mode was fabricated. The C-TENG based on the ZIF-8/MO-PPy@CelF with 31.8% ZIF-8 deposition ratio produced a voltage output of 129 V, a current of 6.8 µA and a transfer charge of 47.4 nC, approximately four times higher than the C-TENG based on ZIF-8/CelF. Moreover, the C-TENG demonstrates excellent cycling stability after continuous operation for 10000 cycles. Our methodology provides insight to further expand the range of positive triboelectric materials available. C-TENG based on ZIF-8/MO-PPy@CelF composite is expected to be used in many applications such as self-powered supercapacitors, sensors and monitors, smart pianos, ping-pong tables, floor mats, etc.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/polym14020332/s1, Figure S1: SEM images of ZIF-8/MO-PPy@CelF at different locations; Figure S2: Current densities of samples; Figure S3: Open circuit voltages at different humidity levels; Table S1: Output performance comparison of C-TENG; Video S1: The LED lights were successfully lit up by C-TENG.
Author Contributions: Q.L. and X.Q. designed the experiments; Q.L. conducted the experiments; Q.L. and X.A. analyzed the data; Q.L. and X.Q. wrote the paper. All authors discussed the results and contributed to the improvement of the final text of the paper. All authors have read and agreed to the published version of the manuscript.
Funding: This work was supported by the National Natural Science Foundation of China (grant no. 31770620).
Institutional Review Board Statement: Not applicable.