Growth of the metal–organic framework ZIF-67 on cellulosic substrates for triboelectric nanogenerators

Abstract

Metal–organic frameworks (MOFs) are porous crystalline materials with a metal ion coordinated to a ligand molecule. Recently, MOFs are being explored extensively for energy harvesting via triboelectrification. However, the majority of MOFs are brittle and hard to grow, thus leading to poor device stability and flexibility. Herein, the growth of ZIF-67 MOF is achieved on a cellulosic filter paper (CFP) and cotton fabric (CF) separately to use as the active layer in a TENG. The grown ZIF-67 MOFs were used for the fabrication of CFP-TENG and CF-TENG in vertical contact separation mode. The CF-TENG device exhibited a high durability with no significant change in the electrical output for a period of 14 000 s. Additionally, the device generated a maximum electrical output of 60 V and 3 μA with an output power density of 5 mW m⁻² at a load resistance of 800 MΩ. The robustness of the MOF grown on cotton fabric was demonstrated by fabricating a contact separation and rotating TENG device. The rotating TENG device produced an output voltage of ∼100 V and current of 3.5 μA, thus confirming the strong adherence of MOFs on the fabric. The CF-TENG was demonstrated for powering electronics via flexible circuits and for biomechanical energy harvesting by utilising finger tapping, hand tapping, jogging and running movements.

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1 Introduction

Metal–organic frameworks (MOFs) are a class of crystalline porous materials with high surface areas, which have been proven to be useful for a wide range of applications, including catalysis, sensing, gas storage, energy harvesting, gas separation and drug delivery.^1–6 Since their invention in 1990, the past two decades has seen an immense growth in MOF development, fabricating around 90 000 MOFs, with almost 500 000 MOF structures predicted.⁷ Among several MOF systems, some of the widely explored MOFs are zeolitic imidazole framework (ZIF) family members, including HKUST, MOF-5, MOF-53, and MIL-88A. ZIF family members have several advantages owing to their unique thermal and chemical stabilities. Recently, MOFs have been used for the fabrication of energy harvesters, such as triboelectric nanogenerators (TENG), in which MOFs were used as active triboelectric layers for self-powered applications.^8,9 TENGs are mechanical energy harvesting devices that can scavenge energy from ambient mechanical energy sources, such as bio-mechanical energy, wind energy, blue energy, and vibration energy. TENGs work on the principle of contact electrification and electrostatic induction, when two triboelectrically active materials come in contact with each other. So far, TENGs have been successfully reported for their applications in powering low-power electronic devices, pressure sensors, wind vector monitoring, position sensors, chemical sensors, drug delivery, self-charging systems, and biomedical applications.^10–15 Several MOFs have been reported as active layers for TENGs, and these include HKUST-1, MIL-88A, and ZIFs.^8,9,16–18

The majority of the MOFs reported for use in TENGs and self-powered applications are synthesised via chemical methods followed by their attachment to the adhesive side of the aluminium or copper tapes.¹⁹ Such devices exhibit poor stability and flexibility, deeming them unsuitable for self-powered applications. One such example is ZIF-67, which was attached on the adhesive side of the copper tape for the fabrication of TENG with Teflon as the opposite layer, producing an output voltage of 70 V and current of 0.9 μA.²⁰ Similarly, several other MOFs, including ZIF-7, ZIF-9, ZIF-10, ZIF-11, ZIF-62, cyclodextrins (α, β and γ form), and several covalent organic frameworks (COFs) have been demonstrated for TENGs using the above method.^21–24 Such devices have limited stability and durability, as the performance of the device decreases with the aging of adhesives present on the aluminium tape. Also, the MOFs started to detach from the aluminium tape upon application of the mechanical force required for TENG operation, leading to poor stability, reduced lifetime, material transfer and poor device reproducibility. In another approach, MOFs have been used as the filler in different polymers to improve the dielectric properties of the polymer, which leads to the enhancement in the TENG output.^21,22,25 One such example is the use of fluorinated MOF as a filler in PDMS for the fabrication of a humidity-resistant TENG.¹⁷ When MOFs are blended with the polymer, the surface property of MOFs do not contribute towards the energy harvesting and the polymer can reduce the MOFs functionality by blocking the access of the pores or channels. In this regard, MOFs are not the preferred choice as they possess low dielectric constant, while ceramic materials like calcium copper titanate (CCTO) and barium titanate (BTO) offer significant advantages due to their high dielectric constant.^26,27 Hence, a well-adhered MOF as an active layer is highly desirable to understand the behaviour of MOFs in energy generation via triboelectric effect.

In this paper, the MOF is directly grown on substrates that are utilised as an active layer for the TENG. ZIF-67 MOF was separately grown directly on cellulosic filter paper (CFP) and cotton fabrics (CF). The growth cycles were fixed for different number of cycles, such as 3-cycles, 5-cycles, and 7-cycles to optimize the amount of MOF crystals grown on the substrates. The in situ grown MOF crystals were structurally analysed using Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The morphological analysis was performed using a field emission scanning electron microscope (FE-SEM). The CFP-TENG device was fabricated using the in situ grown ZIF-67 in vertical contact-separation mode with fluorinated ethylene polymer (FEP) as the opposite layer. The CFP-TENG was fabricated for vertical contact-separation mode and rotary mode to demonstrate the strong adherence of the material on the substrate. The electrical response of the device was analysed separately for the CFP-TENG and CF-TENG devices. The CF-TENG device shows the maximum electrical response of 60 V and 3 μA with the output power density of 5 mW m⁻² at a load resistance of 800 MΩ. Long-term stability tests on both the CFP-TENG and CF-TENG devices demonstrated excellent durability and stability. Capacitor charging analysis was performed to prove that the energy generated from the device is accurate. Similarly, the electrical output of the rotary mode TENG device was analysed and reported to be 300 V_P–P and 6.5 μA_P–P. Finally, the CF-TENG was used to power electronics using flexible circuits, which can be easily attached on the shoe or on the back of an individual without compromising the wearer's comfort for real-time use.

2 Materials and methods

2.1 Materials

2-Methylimidazole, sodium hydroxide (NaOH), sodium chloroacetate (CH₂ClCO₂Na), Whatman filter paper and copper nitrate hexahydrate (Co(NO₃)₂·6H₂O) were purchased from Sigma Aldrich. Methanol was purchased from Daejung Chemical Ltd. Cotton fabric was purchased from the local supermarket.

2.2 Growth of ZIF-67 on filter paper and cotton fabric

The growth of ZIF-67 on filter paper and cotton fabric was achieved by creating anchoring sites on both substrates.²⁸ The carboxymethylation of the filter paper and cotton fabric was carried out using CH₂ClCO₂Na and NaOH. The substrates were dipped in a sodium chloroacetate (1 M) solution prepared in 15 wt% NaOH, followed by stirring for 1 h. The unreacted residues on both substrates were removed by washing in water, and allowed to dry at room temperature. ZIF-67 was grown on the modified filter paper and cotton fabric substrates by dipping them in a methanolic solution of 2-methylimidazole (12.6 mmol) and copper nitrate hexahydrate (1.25 mmol) for 3 h at room temperature. The filter paper and cotton fabric substrates with grown ZIF-67 were removed from the solution and rinsed with methanol to remove the unreacted materials, followed by drying at room temperature.²⁸

2.3 Fabrication of the ZIF-67 TENG device

The TENG was fabricated by bar coating silver ink electrodes on filter paper and cotton fabric. Fluorinated ethylene propylene (FEP) was selected as the negative triboelectric layer. The FEP was sputter-coated with gold. The filter paper-based contact-separation device (CFP-TENG) was prepared by using PDMS as the substrate on both sides and as a spacer (Fig. 2e). The cotton fabric-based contact-separation device (CF-TENG) was prepared by using polytetrafluoroethylene (PTFE) cooking fabric as the substrate, and assembled using spacers. The rotating device was prepared by attaching four FEP sheets on the outer polyethylene terephthalate (PET) layer, followed by folding. The four cotton fabrics were attached on the centre rod. The commercial plastic gears were attached on the outer part using a low-cost 3D pen-printed handle for rotation (Fig. 6a).

2.4 Flexible printed circuit board (fPCB) fabrication

The flexible PCB was fabricated using a single-sided copper 35/00 U copper-clad polyimide film that was 25 μm thick. The required dimensions were cut from the sheet, which was then cleaned with chemicals to remove contaminants. A photoresist layer was applied, and the circuit pattern, designed in EAGLE, was transferred onto the sheet using UV light exposure. Subsequent steps included etching to remove unwanted copper areas and stripping to eliminate the residual photoresist. The board was then thoroughly rinsed with a neutral solution and dried in an oven. Electrical testing was conducted to ensure functionality before mounting the SMD components onto the flexible PCB boards. The first flexible PCB (fPCB) incorporated a bridge rectifier IC and LEDs, with the CF-TENG supplying the necessary power to illuminate the LEDs. The second fPCB featured a capacitor charging circuit in conjunction with a bridge rectifier, powered by the CF-TENG.

2.5 Structural characterization and electrical response

The X-ray diffraction pattern was recorded on a high-resolution XRD instrument from PANalytical at a generator voltage of 40 V and 30 mA tube current. The Alpha II compact FT-IR spectrometer from Bruker was used to acquire the Fourier-transform infrared (FT-IR) spectra. The surface morphology images were captured on a field emission scanning electron microscope (FE-SEM) MIRA-3 from Tescan. A software control linear motor from LinMot Inc. was used to apply force on the devices to induce contact-separation motion (Fig. S3†). The linear motor was mounted on a vibration-free optical table. The acceleration was monitored by software using guide modules of the linear motor. A force gauge from Mecmesin was used to measure the force. The voltage and charge generated by the TENG device were recorded on a Keithley 6514 electrometer. A Stanford SR570 low-noise current pre-amplifier connected to a Tektronix MDO3000 series oscilloscope was used for the current measurements.

3 Results and discussion

Fig. 1 shows the step-by-step growth process of the ZIF-67 MOF on filter paper and cotton fabric. The growth process was carried out in a precursor solution consisting of 2-methylimidazole and Co(NO₃)₂·6H₂O by dipping the carboxymethylated filter paper or cotton fabric in the solution, as schematically shown in Fig. 1a. Fig. 1b shows digital photographic images of ZIF-67 grown directly on the filter paper for different cycle lengths, ranging from 1, 3, 5, and 7 cycles. The FE-SEM analysis shows the morphology of different cycles (1, 3, 5 and 7) of ZIF-67 grown on filter paper (Fig. 1c). The growth of ZIF-67 increases with increased growth cycle lengths, as confirmed by the FE-SEM images. The fibre morphology of the filter paper is clearly visible in the 1-cycle grown ZIF-67, which was then covered by ZIF-67 particles in the subsequent growth cycles. Fig. S1† shows the energy dispersive spectroscopy (EDS) mapping images of the 5-cyc ZIF-67, confirming the presence of C, Co, O and N. The fabricated digital photographic image of the ZIF-67 TENG is shown in Fig. 1d. The MOF was on the filter paper, and FEP was used as the triboelectric layers with silver and gold as the respective electrodes, which were housed with PDMS substrate to fabricate the contact-separation based TENG device. Fig. 2a shows the FT-IR spectra of the carboxymethylated filter paper (CMP) and 1-cycle grown ZIF-67 on CMP. The peaks at 1374 cm⁻¹ and 1314 cm⁻¹ resulted from C–H deformation and CH₂ wagging in CMP, respectively.²⁹ The peaks at 1160 cm⁻¹ and the 988–1060 cm⁻¹ region are attributed to the symmetrical stretching vibration of C–O–C and the C–O stretching vibrations in cellulose, respectively. The additional peaks at 1143 cm⁻¹ and 754 cm⁻¹ in CMP/ZIF-67 are attributed to ZIF-67.²⁸Fig. 2b shows the FT-IR spectra of 2-cycles grown ZIF-67, which matches well with the literature, and confirms the successful growth of ZIF-67. The peaks at 1579 cm⁻¹ and 1423 cm⁻¹ are ascribed to the C=N stretching mode and the stretching vibrations in the imidazole ring, respectively. The peaks at 1173 and 1140 cm⁻¹ result from the non-planar vibrations. The peaks at 992 cm⁻¹ and 755 cm⁻¹ are due to the plane-bending vibrations and out-of-plane bending of the 2-methylimidazole ring.^28,30–32Fig. 2c shows the XRD pattern of CMP and 1 cycle CMP/ZIF-67. The peaks at 15° and 22° are due to cellulose I, as it is the major component of the filter paper.³³ The peaks at 7.4°, 10.5°, 12.8° and 18.17° correspond to the (011), (002), (112) and (222) planes of ZIF-67, respectively.^34,35 ZIF-67 comprises Co²⁺ metal ions and 2-methylimidazole as a linker arranged in a cubic crystal symmetry, as shown in Fig. 2d.³⁶ The layer-by-layer schematic of the ZIF-67 TENG is shown in Fig. 2e with the active layers and electrodes shown in different colours, and the device structure is shown in Fig. 2f.

Fig. 1 Growth process of ZIF-67 TENG with morphological analysis. (a) Step-by-step growth process of the ZIF-67 on cellulosic filter paper and cotton fabric. (b) Digital photographic image of the device grown on the filter paper for different cycles, such as 1-cyc, 3-cyc, 5-cyc and 7-cyc. (c) FE-SEM morphology of the grown ZIF-67 for 1-cyc, 3-cyc, 5-cyc and 7-cyc, respectively. (d) Digital photographic image of the TENG device fabricated using the ZIF-67 MOF and FEP as active layers separated by a PDMS spacer.

Fig. 2 Structural characterization of ZIF-67 and schematic of the ZIF-TENG. (a) FTIR spectra of CMP and 1-cyc grown ZIF-67 on CMP, (b) FT-IR spectra of 2-cyc grown ZIF-67 on CMP. (c) XRD pattern of CMP and 1-cyc grown ZIF-67 on CMP. (d) Crystal structure of the synthesised ZIF-67 MOF with the respective molecules and its colour representations. (e) Layer-by-layer schematic of the fabricated triboelectric nanogenerator with the clear representation of layers in the respective colours. (f) Schematic of the device with the triboelectric layers assembled for contact-separation mode with spacers.

The working mechanism and electrical response of the TENG device fabricated using cellulosic filter paper (represented as CFP-TENG) is shown in Fig. 3. The CF-TENG works as a contact-separation mode TENG device that operates via contact electrification and electrostatic induction. When the two triboelectric layers, i.e., ZIF-67 and FEP, are in contact with each other, the two friction layers are in the equilibrium condition with no charge transfer, as shown in Fig. 3a(i). When the layers start to separate, the electrons start flowing from one electrode to the other, leading to the generation of instantaneous electrical current, i.e., first half cycle of the TENGs AC output, as shown in Fig. 3a(ii). The third step of the mechanism is complete separation of triboelectric layers, leading to an equilibrium condition. Next, the active layers approach each other, leading to the transfer of charges in the opposite direction, resulting in another half cycle of the AC output. Fig. S2† depicts the device in the full contact, separating, separated, and contacting stages, corresponding to the devices working mechanism. Fig. 3b and c show the electric output voltage and current for the CFP-TENG grown for 3, 5 and 7 cycles, with the 5 cycles grown device exhibiting a higher electrical response of 50 V and 400 nA, compared to the 3-cycles and 7-cycles grown ZIF-67. This phenomenon clearly shows that the 3-cycle grown device has insufficient material adhered on the substrate, resulting in less electrical output. Furthermore, the 7-cycle grown device has a higher number of particles and a thicker layer, resulting in less charge transfer from the layer to the electrode. In the 5-cycle grown TENG device, the ZIF-67 growth is optimum for effective contact, resulting in a higher potential compared to the other two growth options. Fig. 3d shows the open circuit voltage of the CFP-TENG device, and the transferred charge is shown in Fig. 3e for all three CFP-TENG growth cycle protocols. The 5-cycle CFP-TENG produced an open circuit voltage (V_OC) of 55 V and transferred charge of 12 nC. The transferred charge profile of all 3 CFP-TENG cycle types matches well with the corresponding voltage and current output.

Fig. 3 Working mechanism and electrical output response of the CFP-TENG device. (a) Contact-separation working mechanism of the CFP-TENG device, showing the equilibrium and power generation stages of the contact electrification. (b) and (c) Voltage and current output of the CFP-TENG device grown for 3, 5 and 7 cycles. (d) Open circuit voltage of the CFP-TENG device, where ZIF-67 was grown for 3, 5 and 7 cycles. (e) Transferred charge between the triboelectric layers of ZIF-67 and FEP grown at different growth cycles of 3, 5, and 7 cycles.

The electrical response of the CFP-TENG device was studied extensively, and the corresponding electrical response is shown in Fig. 4. The impedance matching analysis is important for analysing the optimum load matching resistance for the generation of electrical output response, which is shown in Fig. 4a. The device shows a high output power of 1.2 μW and a power density of 8 mW m⁻² at 800 MΩ. Fig. 4b shows the voltage response of the CFP-TENG device under various load resistors. The endurance of the device was tested to analyse the maximum duration the device operates with stable output delivery – this was recorded for a period of 2000 s at a force of 10 N and frequency of 1 Hz. The response shows that the device is stable and able to power continuously without any interruption in its response for the entire period at which the test is done, as shown in Fig. 4c. Extending the performance analysis of the device, Fig. 4d shows the capacity of the CFP-TENG powering commercial capacitors with a rating of 0.22 μF and 1 μF. The capacitors rated 0.22 μF and 1 μF charge to a voltage of 3 V under vertical contact separation motion, where the 0.22 μF capacitor reaches 3 V in 60 s and the 1 μF capacitor reaches 3 V charging in 290 s, validating the ability of the device to charge low-power electronic devices. Fig. 4d shows the capability of the CFP-TENG device to charge a 1 μF commercial capacitor under different operating accelerations of the linear motor, such as 3 m s⁻², 5 m s⁻², and 7 m s⁻², respectively, for 3 V. The result shows that the device responds to operating frequencies with higher acceleration, leading to the generation of more charges, which transfers quickly between the layers to generate a higher response.

Fig. 4 Detailed electrical response analysis of the CFP-TENG device. (a) Impedance matching analysis of the CFP-TENG device analysed across various load resistances showing the output power and power density of the device. (b) Voltage output of the device across various load resistances. (c) Stability analysis of the CFP-TENG device operated continuously for 2000 s, in which the inset shows the peak pattern of the device working stably without interruption. (d) Capacitor charging data showing the capacitor rating for 0.22 μF and 1 μF, reaching the voltage value of 3 V. (e) Charging 1 μF capacitor under various accelerating conditions, such as 3 m s⁻², 5 m s⁻², and 7 m s⁻².

Fig. 5 shows the electrical response of the TENG device fabricated using the ZIF-67 grown on cotton fabric (represented as CF-TENG). The device shows a maximum open circuit voltage (V_OC) of 60 V and short circuit current of 3 μA peak-to-peak, which are shown in Fig. 5a and b, respectively. The transferred charge for the device is around 13 nC when fabricated using the cotton fabric, as shown in Fig. 5c. For validating the performance of the CF-TENG device, a switching polarity test was done, which confirmed the clear phase shift in the voltage output response (Fig. 5d). Extending the performance analysis of the CF-TENG, a durability test was conducted for a period of 14 000 s, showing that the device can operate for a long period of time without interruption in its electrical response, as shown in Fig. 5e. The inset shows the peak pattern at the later stages and confirms the uniformity of the peaks. The voltage response of the device was analysed using various load matching resistances-see Fig. 5f. The impedance matching analysis was performed by matching the impedance of the device with various load resistance values, ranging from ohms to giga ohms. The device has achieved a maximum peak power of 3 μW and power density of 5 mW m⁻², as shown in Fig. 5g. The CF-TENG device is then used to power commercial capacitors of 0.22 μF, 1 μF and 2 μF to a voltage of 3 V. The capacitor with high rating (2 μF) reaches the 3 V limit in 250 s, as shown in Fig. 5h. The 2 μF capacitor is then used to check its charging capability powered with the CF-TENG device under various accelerations. The acceleration value of 3 m s⁻² charges the 2 μF capacitor quickly in 100 s, which is due to the high electrical output generation under high accelerating conditions compared with the lower acceleration values, as shown in Fig. 5i.

Fig. 5 Electrical response analysis of the CF-TENG device. (a) Open circuit voltage of the CF-TENG device. (b) Short circuit current of the fabricated TENG device. (c) Charge transfer between the triboelectric layers of the CF-TENG device. (d) Switching polarity test showing the exact phase shift of the electrical signal upon switching the connections. (e) Stability test of the device showing the long term stability for the period of 14 000 s with the inset showing the peak pattern for the stability of the device. (f) Voltage output of the device under various load resistances. (g) Load matching analysis of the device under various load resistances, showing the power and power density of the device. (h) Capacitor charging studies for the fabricated CF-TENG device with the capacitor values of 0.22 μF, 1 μF, and 2 μF. (i) Charging of the 2 μF capacitor under various accelerations, such as 0.5 m s⁻², 1 m s⁻², 2 m s⁻² and 3 m s⁻².

Extending the analysis of the contact-separation-based TENG through its charge transfer mechanism and electrical response analysis, a rotating TENG device was also designed to analyse the performance of the ZIF-67 TENG device, as shown in Fig. 6a. The device is fabricated by using a thick PET sheet rolled in a cylindrical structure, which houses the ZIF-67 grown layers as triboelectric materials on the central rotating rod and FEP on the wall of the cylinder. A rotating mechanism is arranged on the top of the cylinder using gear wheels and a handle, which is connected to a rotor inside the cylinder. The contact electrodes are kept outside to establish electrical contacts to proceed for applications, as well as analysis. The working mechanism of the sliding mode ZIF-67 TENG device is shown in Fig. 6b. The sliding mode working mechanism is operated in four stages: in Stage 1, both ZIF-67 and FEP layer overlap each other in the equilibrium condition. Under the equilibrium condition, there is no separation of the changed surfaces. As a result, the net electrical potential across the two electrodes is zero, leading to no electrical output. When the FEP layer starts to slide, a potential difference is generated. The electrons start to flow from the top electrode to the bottom electrode, leading to generation of an electrical output, as shown in Stage 2. At the third stage, the FEP layer completely slides away with no friction on the ZIF-67 layer kept static. There is no flow of electrons between the layers and the output current is zero. At the fourth stage, the FEP layer starts to slide in the direction of the ZIF-67 layer towards gradual overlap. The friction generates the electrical output along with the flow of electrons from the bottom electrode to the top electrode by the generation of the electrical output, in which the direction of the current is reversed. The device generates a maximum electrical response of approximately 300 V peak-to-peak and the current output of 6.5 μA, respectively (Fig. 6c and d). The device shows excellent switching polarity test results, which show the 180° phase shift in its electrical response, proving the generation of an electrical output from the device (Fig. 6e). The above analysis shows that the design of the rotating TENG using the grown ZIF-67 MOF material is feasible, and ZIF-67 can be a promising candidate for rotating TENGs due to its excellent adherence on the substrate.

Fig. 6 Fabrication, working mechanism and electrical response of the rotating TENG device. (a) Digital photograph of the fabricated rotary TENG device. (b) Linear sliding mode working mechanism of the rotary TENG device. (c) and (d) Voltage and current output response of the fabricated rotary mode TENG device. (e) Switching polarity test of the fabricated rotating mode TENG device.

Fig. 7 shows the self-powered applications and biomechanical energy harvesting performance of the CF-TENG. A flexible printed circuit board (fPCB) comprising a switch and rectifier was fabricated for powering the emitting diodes (LEDs) (Fig. 7a, top). The fPCB is flexible, and thus can be easily used with wearables without any discomfort (Fig. 7a). The bottom part of Fig. 7a shows a rectification charging circuit to convert the AC output of CF-TENG to DC output for its storage in the capacitor. The circuit on the fPCB consists of four different capacitors of capacitance 10 μF, 47 μF and 100 μF, which can be connected with the rectified output by turning the switch on. Fig. 7b shows an array of LEDs glowing using the shaker and by the jogging movements of a human. The fPCB was attached on the back of the individual for powering using jogging motion. Such self-powered fPCBs can potentially be used during night-time hours by athletes and for exercise purposes. The fPCB rectification charging circuit was attached on the shoes to charge the 10 μF capacitor by jogging (Fig. 7c). Finally, several biomechanical motions (including finger tapping, hand tapping, jogging and running) were utilised for generating electrical output using the CF-TENG. The jogging and running generated a higher output compared to the finger and hand tapping due to the higher force and frequency.

Fig. 7 Self-powered applications and biomechanical energy harvesting performance of CF-TENG. (a) Flexible printed circuit board (fPCB) for powering light emitting diodes (LEDs) under planar and bending conditions (top) and flexible rectification circuit with three different capacitors of 10, 47 and 100 μF connected via switch (bottom). (b) LEDs on fPCB powered using shaker and jogging (attached on the back of an individual). (c) Charging of 10 μF capacitor on the flexible rectification circuit attached on the shoe by utilising mechanical energy generated from jogging. (d) Biomechanical energy harvesting performance of CF-TENG using finger tapping, hand tapping, jogging and running movements.

4 Conclusions

In summary, this paper reports the fabrication and analysis of a TENG made of in situ grown ZIF-67 on cellulosic filter paper and cotton as substrates. The reported work overcomes the drawback in the previous report involving the coating of MOFs on adhesive tape for the electrical analysis, which showed an inaccurate electrical response. The synthesised ZIF-67 has been used for the fabrication of both contact-separation and sliding mode triboelectric nanogenerators. The in situ grown ZIF-67 was structurally characterised using FTIR spectroscopy, X-ray diffractometers, and field emission scanning electron microscopy. The working mechanism of both contact and separation mode, as well as sliding mode, was studied, and the electrical responses were measured. The device generates a maximum electrical response of 50 V and 400 nA, and an output power of 1.2 μW with power density of 8 mW m⁻² (at 800 MΩ). This occurred for the device with the MOF layer grown on cellulosic paper over 5 growth cycles, which was higher than the output corresponding to 3 and 7 cycles of growth. Similarly, the MOF grown on CFP generates a higher power density compared to the MOF layer grown on CF. The devices exhibited good stability and the ability to power up various commercial capacitors. In addition, a rotary type of device was made to check the performance of the ZIF-67 device under rotary conditions, in which the device was proved to generate higher electrical output. Finally, the device was demonstrated for self-powered applications, including LEDs lighting and capacitor charging via flexible PCBs. The device can successfully utilise numerous biomechanical motions, like finger tapping, hand tapping and jogging, to generate energy.

Data availability

The data that support the findings of this study are included in the manuscript.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the Basic Science Research Program (2023R1A2C3004336) & Regional Leading Research Center (RS-2024-00405278) through the National Research Foundation of Korea (NRF) grant funded by the Korean government. Support is also acknowledged from the Engineering and Physical Sciences Research Council (EPSRC) in the UK through the ‘NextGenT-TENG’ Standard Grant (EP/V003380/1).

References

1. M. Kalaj and S. M. Cohen, ACS Cent. Sci., 2020, 6, 1046–1057.
2. H.-C. J. Zhou and S. Kitagawa, Chem. Soc. Rev., 2014, 43, 5415–5418.
3. H. Furukawa, K. E. Cordova, M. O'Keeffe and O. M. Yaghi, Science, 2013, 341, 1230444.
4. J. F. Olorunyomi, S. T. Geh, R. A. Caruso and C. M. Doherty, Mater. Horiz., 2021, 8, 2387–2419.
5. J. Wu, J. Chen, C. Wang, Y. Zhou, K. Ba, H. Xu, W. Bao, X. Xu, A. Carlsson, S. Lazar, A. Meingast, Z. Sun and H. Deng, Adv. Sci., 2020, 7, 1903003.
6. J. F. Olorunyomi, M. M. Sadiq, M. Batten, K. Konstas, D. Chen, C. M. Doherty and R. A. Caruso, Adv. Opt. Mater., 2020, 8, 2000961.
7. S. M. Moosavi, A. Nandy, K. M. Jablonka, D. Ongari, J. P. Janet, P. G. Boyd, Y. Lee, B. Smit and H. J. Kulik, Nat. Commun., 2020, 11, 4068.
8. G. Khandelwal, M. K. Ediriweera, N. Kumari, N. P. Maria Joseph Raj, S. K. Cho and S.-J. Kim, ACS Appl. Mater. Interfaces, 2021, 13, 18887–18896.
9. G. Khandelwal, A. Chandrasekhar, N. P. Maria Joseph Raj and S.-J. Kim, Adv. Energy Mater., 2019, 9, 1803581.
10. G. Khandelwal and R. Dahiya, Adv. Mater., 2022, 34, 2200724.
11. J. Luo and Z. L. Wang, Energy Storage Mater., 2019, 23, 617–628.
12. G. Khandelwal, N. P. Maria Joseph Raj and S.-J. Kim, Nano Today, 2020, 33, 100882.
13. M.-Z. Huang, P. Parashar, A.-R. Chen, S.-C. Shi, Y.-H. Tseng, K. C. Lim, H.-Y. Yeh, A. Pal, D.-Y. Kang and Z.-H. Lin, Nano Energy, 2024, 122, 109266.
14. D. Choi, Y. Lee, Z.-H. Lin, S. Cho, M. Kim, C. K. Ao, S. Soh, C. Sohn, C. K. Jeong, J. Lee, M. Lee, S. Lee, J. Ryu, P. Parashar, Y. Cho, J. Ahn, I.-D. Kim, F. Jiang, P. S. Lee, G. Khandelwal, S.-J. Kim, H. S. Kim, H.-C. Song, M. Kim, J. Nah, W. Kim, H. G. Menge, Y. T. Park, W. Xu, J. Hao, H. Park, J.-H. Lee, D.-M. Lee, S.-W. Kim, J. Y. Park, H. Zhang, Y. Zi, R. Guo, J. Cheng, Z. Yang, Y. Xie, S. Lee, J. Chung, I.-K. Oh, J.-S. Kim, T. Cheng, Q. Gao, G. Cheng, G. Gu, M. Shim, J. Jung, C. Yun, C. Zhang, G. Liu, Y. Chen, S. Kim, X. Chen, J. Hu, X. Pu, Z. H. Guo, X. Wang, J. Chen, X. Xiao, X. Xie, M. Jarin, H. Zhang, Y.-C. Lai, T. He, H. Kim, I. Park, J. Ahn, N. D. Huynh, Y. Yang, Z. L. Wang, J. M. Baik and D. Choi, ACS Nano, 2023, 17, 11087–11219.
15. C.-T. Chen, C.-C. Weng, K.-P. Fan, S. R. Barman, A. Pal, C.-B. Liu, Y.-K. Li, Z.-H. Lin and C.-C. Chang, ACS Appl. Mater. Interfaces, 2024, 16, 1502–1510.
16. G. Khandelwal, N. P. Maria Joseph Raj, V. Vivekananthan and S.-J. Kim, iScience, 2021, 24, 102064.
17. R. Wen, J. Guo, A. Yu, J. Zhai and Z. l. Wang, Adv. Funct. Mater., 2019, 29, 1807655.
18. H.-Z. Ma, C. Luo, J.-N. Zhao, Y. Shao, Y.-H. Zhang, X. Liu, S. Li, B. Yin, K. Zhang, K. Ke, L. Zhou and M.-B. Yang, ACS Appl. Mater. Interfaces, 2023, 15, 37563–37570.
19. Y.-M. Wang, X. Zhang, Y. Ran, C. Liu, D. Wang, G. Mao, X. Jiang, S. Wang, X.-B. Yin and R. Yang, ACS Mater. Lett., 2024, 6, 3883–3898.
20. S. Hajra, M. Sahu, A. M. Padhan, J. Swain, B. K. Panigrahi, H.-G. Kim, S.-W. Bang, S. Park, R. Sahu and H. J. Kim, J. Mater. Chem. C, 2021, 9, 17319–17330.
21. R. K. Rajaboina, U. K. Khanapuram, V. Vivekananthan, G. Khandelwal, S. Potu, A. Babu, N. Madathil, M. Velpula and P. Kodali, Small, 2024, 20, 2306209.
22. G. Khandelwal, N. P. Maria Joseph Raj and S.-J. Kim, Adv. Energy Mater., 2021, 11, 2101170.
23. G. Khandelwal, N. P. Maria Joseph Raj and S.-J. Kim, J. Mater. Chem. A, 2020, 8, 17817–17825.
24. G. Khandelwal, N. P. Maria Joseph Raj and S.-J. Kim, Adv. Funct. Mater., 2020, 30, 1910162.
25. Z. Abbas, M. Anithkumar, A. P. S. Prasanna, N. Hussain, S.-J. Kim and S. M. Mobin, J. Mater. Chem. A, 2023, 11, 26531–26542.
26. J. Kim, H. Ryu, J. H. Lee, U. Khan, S. S. Kwak, H.-J. Yoon and S.-W. Kim, Adv. Energy Mater., 2020, 10, 1903524.
27. J. Wang, H. Wu, Z. Wang, W. He, C. Shan, S. Fu, Y. Du, H. Liu and C. Hu, Adv. Funct. Mater., 2022, 32, 2204322.
28. J. Park and M. Oh, Nanoscale, 2017, 9, 12850–12854.
29. R. Ou, Y. Xie, X. Shen, F. Yuan, H. Wang and Q. Wang, J. Mater. Sci., 2012, 47, 5978–5986.
30. Z. Wang, D. Ren, H. Yu, S. Jiang, S. Zhang and X. Zhang, Biotechnol. Rep., 2020, 28, e00553.
31. M. M. Hulagabali, A. V. Sonawane and G. R. Vesmawala, J. Mater. Civ. Eng., 2023, 35, 04023090.
32. J. Zhao, Front. Mater., 2021, 8, 800820.
33. Z. Li, N. Hori and A. Takemura, Cellulose, 2020, 27, 6537–6547.
34. X. Guo, T. Xing, Y. Lou and J. Chen, J. Solid State Chem., 2016, 235, 107–112.
35. M. Yaghoubi, A. R. Zanganeh, N. Mokhtarian and M. H. Vakili, J. Appl. Electrochem., 2022, 52, 683–696.
36. G. Zhong, D. Liu and J. Zhang, J. Mater. Chem. A, 2018, 6, 1887–1899.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nr03909b

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