Synergy Unleashed: Piezo–Tribo Hybrid Harvester for Sustainable Power Generation Toward Augmented and Virtual Reality Applications

Recently, piezo–tribo hybrid nanogenerators are developed using the synergistic effect. Exploration of multi‐crystalline piezoelectric materials adds the advantage of mechanically to electrically converting hybrid systems. On the way, the multi‐crystalline 0.3Ba0.7Ca0.3TiO3–0.7BaSn0.12Ti0.88O3 (BCST) materials are characterized by the structural, vibrational, and ferroelectric properties. The synergistic effect‐based polydimethylsiloxane/BCST hybrid composite film is prepared for multi‐energy harvesting. The triboelectric nanogenerator produces a maximum output voltage of 210 V. The hybrid device obtains the maximum output performance of 326 V. The harvested energy is used to charge commercial capacitors and power low‐power electronics. The signal processing and analysis unit is constructed for monitoring or functioning in real‐time finger movement. In the future, this kind of system can be coupled with augmented and virtual reality applications.


Introduction
Augmented reality (AR) and virtual reality (VR) are immersive technologies that have gained significant applications in recent years.Using this technology provides immense applications in education, gaming, and healthcare.AR and VR technology requires hardware (sensors, processing units, or interfaces) and software (to analyze and function).In recent decades, a self-powered sensor was developed to generate energy for the operation and convert the input trigger into electric signals.The mechano-electric conversion sensor was developed using triboelectric and piezoelectric properties.A triboelectric nanogenerator (TENG) is a type of energy harvester that generates electricity using mechanical energy (biomechanical motions, [1] water waves, [2] vibrations, [3] and rainwater [4] ).The fundamentals of triboelectrification can be explained with the coupled effect of contact electrification and electrostatic induction.Electron-and ion-transfer models were developed to understand the charge transfer or generation during contact electrification. [5]The wide range of material choices [6] and fundamental modes of TENG paves various applications, including healthcare, [7] physical sensors, [8] chemical sensors, [9] sports, [10] wireless transmission, [11] self-powered sensors, [12] and smart applications. [13]However, the output performance of the TENG needs to be improved.Adding filler materials into the triboactive layers is an effective process for output performance improvement.Metal oxides, [14] metal organic frameworks, [15] graphene, [16] graphene oxide, [17] and 2D materials [18] were used as the fillers.Even though these fillers help to improve the performance, the sensing ability (mechanical to electric signal conversion) can be improved by the mechanical sensitive fillers.In recent years, piezoelectric materials have been used as fillers to obtain the synergistic effect of piezo-tribo electrification to improve the sensitivity of the nanogeneratorbased sensors.
Piezoelectric materials are a typical class of crystalline materials with non-centro symmetric crystal structures, with the center of positive and negative charge, which do not coincide.Usually, perovskites-and wurtzite-based ceramic materials were active piezoelectric materials.The lead-based materials provide high piezoelectric effects.Due to environmental toxicity, lead-free materials are required.These pave the path to the new type of ceramics (barium titanate (BaTiO 3 ), [19] potassium sodium niobate, [20] sodium bismuth titanate [21] ) and polymer poly (vinylidene fluoride)- [22] and poly(L-lactic acid) [23] -based materials.The 0.3Ba 0.7 Ca 0.3 TiO 3 -0.7BaSn0.12 Ti 0.88 O 3 (BCST) is a derived system from the BaTiO 3 parental system.As piezoelectricity depends on the crystal property of the materials, BCST crystal phases were adjusted with the sintering temperature, dopant concentration, and pressure applied to the pellets.Minor orthorhombic and rhombohedral phases were developed along with the significant tetragonal phase.The prepared BCST film obtained the maximum remnant polarization of 8.9189 μC cm À2 .
The technologies like AR and VR require good sensing capabilities.Coupling the mechanical-dependent triboelectric and piezoelectric nanogenerators (PENGs) leads to improved sensing ability.In this work, synergistic piezo-tribo hybridization was carried out using the piezoelectric material as a filler in the triboelectric film.The multifunctional polydimethylsiloxane (PDMS)/BCST hybrid composite (PBC) film was used to prepare TENG and PENG.The output performance of TENG and PENG were systematically studied.PB-TG device produced a peak-topeak output voltage of 210 V, and the current density of 1.8 mA m À2 is due to the piezoelectric and triboelectric synergistic effect.PB-PG device obtained a maximum output voltage of 35 V and a current density of 0.45 mA m À2 .Hybridized electrical connections were made for the hybridized output of PB-TG and PB-PG.The integrated hybrid system produced the maximum rectified voltage of 326 V.The harvested energy was utilized to charge commercial capacitors and power low-power electronics (light emitting diode (LED), hygrometer, calculator).Signal processing and analyzing (SPA) units were constructed to study the motion of the fingers.Various bending states of the finger were analyzed.

Discussion
ABO 3 is a perovskite-type crystal system with the metal ions at A and B sites.In the BCST multiphase crystal system, Ba 2þ (1.61 Å) or Ca 2þ (1.36 Å) ions were located at the A sites, and B sites were placed with Ti 4þ (0.605 Å) or Sn 4þ (0.69 Å) ions.The change of ionic radius in the A and B site atoms of the perovskite due to the replacement of dopant leads to lattice distortion, which favors the phase changes.The sintering temperature and load applied to the pellets support the phase transformation from tetragonal (P4mm) to orthorhombic (Amm2) phase.Figure 1a shows the XRD analysis of BCST.XRD pattern conforms to the predominant tetragonal phase.The peak splitting at around 45°and 56°c onfirms the existence of orthorhombic phases.Figure S1a, Supporting Information, shows the peak splitting around 45°.The plane (002) belongs to the tetragonal and (020) plane to the orthorhombic.The orthorhombic phases plane (102) was identified around 56°, as shown in Figure S1b, Supporting Information. [19,24]The degree of ionic mixing has been evaluated using Rietveld refinement, and the values of quantified atomic occupancies and lattice parameters of the dopants are mentioned in Table S1, Supporting Information.The results show the existence of the tetragonal system (space group P4mm, 68%) and orthorhombic system (space group Amm2, 32%).The multi-crystalline BCST local phonon modes were measured using the Raman spectra, as shown in Figure 1b.The spectrum reveals distinctive characteristics: a peak between 185 and 192 cm À1 signifies the presence of the longitudinal optical phonon vibration mode associated with the orthorhombic phase.The broad peak ranging from 210 to 260 cm À1 indicates the transition between the tetragonal and orthorhombic phases.The longitudinal and transverse vibrations of the tetragonal phase were observed at 298-302 cm À1 . [19,25]he ferroelectric analyses were analyzed at room temperature at 10 Hz frequency.The hysteresis loop and current-field (I-E) curve are shown in Figure 1c.The sample shows higher remnant polarization (P r = 8.9189 μC cm À2 ), saturation polarization (P s = 22.0864 μC cm À2 ), and lower coercive field (E c = 3.0202) was observed, which indicates good sample nature.The I-E curve shows two types of current signals: leakage current and peak current due to domain switching.The curve confirms the switchable domain peak current in the I-E curve.The domain peak current magnitude increases with the increase in the electric field and is saturated at the end.Usually, the leakage current can be observed at the maximum applied field. [26]he magnitude of the leakage current is low, as shown in Figure 1c.The coercive field at the hysteresis loop and I-E curve seem almost equal.Figure 1d shows the hysteresis loop with the increased electric field varying from 5 to 50 kV.The remnant polarization increases with the increase of the electric field, as in Figure 1e.It is quite worth noting that the coercive field is less than 5 kV, even over 50 kV of the applied electric field, as shown in Figure 1f.The surface morphology and elemental spectrum were analyzed using the SEM. Figure S3, Supporting Information, shows the surface image and elemental analysis of the BCST pellet.The surface morphology of the pellet shows good grain growth obtained.Elemental analysis confirms the formation of BCST. Figure S4, Supporting Information, shows the SEM image of BCST particles, and randomly oriented particles were identified.
The PBC film was prepared using spin-coating techniques for energy-harvesting studies.Figure 1g shows the XRD analysis of PDMS and PBC film.The hump around 10°-15°is attributed to the amorphous nature of the film.The remaining crystalline peaks belong to BCST.Raman spectra of pure PDMS and PBC film are shown in Figure 1h.The predominant peak of PDMS was observed at 2700-3000 cm À1 .The local phonon modes of BCST were observed at 150-800 cm À1 .Figure S5, Supporting Information, shows that the surface morphology of the PDMS is smooth, and elemental analysis conforms to the existence of silicon, oxygen, and carbon.Figure S6, Supporting Information, shows the cross-sectional view of the hybrid composite film.The SEM image shows that BCST particles were loaded into the polymer matrix.Due to the filler loading, roughness is increased.Energy-dispersive X-ray analysis mapping confirms the presence of BCST in the hybrid composite film, as shown in Figure S7, Supporting Information.
The surface roughness of the pure and hybrid composite film was characterized using an atomic force microscope (AFM), as shown in Figure S8, Supporting Information, and Figure 1i.The pure PDMS film has an average roughness (R a ) of 2 nm.Due to the particle loading, the hybrid had an enhanced average roughness (R a ) of 7.45 nm.The nature of fiber formation and polymerization in pure PDMS film differs from hybrid composite film.The surface roughness of the composite film can be affected by various factors, including the filler concentration, size and shape of the fillers, and the orientation of the fibers.Figure S9, Supporting Information, shows the dielectric constant of pure PDMS and hybrid composite film.Due to the filler loading, the dielectric constant was improved for the hybrid composite film.
The energy-harvesting ability of the hybrid composite films was systematically studied for piezoelectric, triboelectric, and hybrid harvesting.The TENG was prepared using the hybrid composite film and copper as the active layer for the vertical contact separation mode, as shown in Figure 2a.A schematic representation of the TENG device mechanism is shown in Figure S10, Supporting Information.The potential difference between the electrodes causes the current flow during the pressing and releasing state.The charges on the layer and electrode create the dipole orientation of the fillers.The dipole orientation further helps to increase the triboelectrification. Figure 2b,c shows the output performance of pure PDMS and hybrid composite film studied under an applied force of 2 N (approximately 0.7 Hz of frequency).The PBC-film-based device (PB-TG) shows a maximum output voltage of 210 V, which is 2.5 times higher than the PDMS devices.And the current density is also higher for PB-TG devices.This enhancement in the output is due to improved roughness, dielectric property, and surface charge.Roughness eventually increases the number of contact points for contact electrification.BCST is a ceramic-based material with a high dielectric constant, which enhances the dielectric property of the composite film.The fillers used in the composite film can create filling defects, change dipoles, change permittivity, and charge transfer between the polymer and fillers, causing an enhancement in the surface charge.Also, the coupling effect of piezo and tribo can account for the output performance due to the piezoelectric filler in the composite film.Figure 2d shows the stability test of the PB-TG and found that the devices were stable over 3600 s on the continuous mechanical stimuli.
The load-matching analysis of the PB-TG device was carried out.Over 70 MΩ resistance, the devices obtained a peak power of 140 μW and power density of 233 mW m À2 , as shown in Figure 2e.The reported TENG was tested with different applied frequencies, as shown in Figure 2f. Figure S11, Supporting Information, shows the output performance of the different dimension PB-TG devices.The ability of the PB-TG was analyzed using commercial capacitor charging.PB-TG charged the capacitor of 1, 4.7, and 10 μF of 13, 4.65, and 2.17 V, respectively, at 300 s, as shown in Figure S12a, Supporting Information.The Q-E curve shows the charge and energy stored in the capacitor (1 μF) shown in Figure S12b, Supporting Information.The charge and energy stored at different intervals are shown in Figure S12c, Supporting Information.
Figure 3a shows the layer-by-layer schematic of the PENG device prepared using the hybrid composite film.The PENG was named as PB-PG.Figure S13, Supporting Information, shows the schematic of the PENG device mechanism.Before the mechanical stimuli, the net dipole moment is zero.The stress-induced dipole orientation produces a forward current during the applied force condition.On the force-removed condition, the orientation of the dipoles tends to align with the initial condition, which causes the reserve current flow.The device obtained a peak-to-peak output voltage of 35 V and a current density of 0.45 mA m À2 as shown in Figure 3b,c, with an applied force of 2 N. PDMS is a piezoelectric polymer, and the obtained output performance is due to the piezoelectric filler used in the composite film.The stability was analyzed for the PB-PG device with continuous mechanical stimuli under 1 m s À2 acceleration.Figure 3d shows that the device is stable without any noticeable deformation in the output generation.Figure S14, Supporting Information, shows the output performance of PB-PG devices with different dimensions.The load-matching analysis shows the peak power density of 0.72 μW and power density of 1.2 mW m À2 over the load resistance of 100 MΩ, as shown in Figure 3e.Varying the acceleration (0.5-5 m s À2 ) piezoelectric response were studied.The piezoelectric output performance increased with the increase in acceleration, as shown in Figure 3f.The PB-PG devices were tested with commercial capacitor charging, as shown in Figure S14a, Supporting Information.The devices charged 0.22, 0.47, and 1 μF capacitors of 10.68, 5.61, and 2.39 V in 300 s.The charge and energy stored in the commercial capacitor (1 μF) are shown in Figure S14b, Supporting Information.Figure S14c, Supporting Information, shows the calculated values of energy and charge stored in a capacitor.
Figure 4a shows the layer-by-layer scheme of the integrated hybrid device and electrical connection to obtain the hybridized output shown in Figure S15, Supporting Information.The hybridized nanogenerator was obtained by integrating triboelectric (PB-TG) and PENG (PB-HG), named H-PB.With the integrated structure and hybridized electrical connection, the H-PB hybrid device obtained a rectified voltage of 326 V, which is higher than the rectified voltage of the reported TENG and PENG devices, as shown in Figure 4b with an applied force of 2 N.After the double rectification, the piezoelectric and triboelectric outputs were hybridized, which eventually caused the improvement of the H-PB device.The capacitor charging of the integrated H-PB hybrid device shows a charged voltage of 15 V in 300 s for a 1 μF capacitor, as shown in Figure 4c.The hybrid nanogenerator's charge-and energy-storing behaviors  were analyzed using Figure 4d.The charge-and energy-storing rate is comparatively higher with the TENG and PENG devices, further confirming the enhancement due to the hybridization.Figure 4e shows the charge and energy stored at different time intervals using the integrated hybrid system.Powering lowpower electronics like LEDs, hygrometers, and calculators were tested to demonstrate the integrated H-PB hybrid device's effectiveness.The hybrid device powered 30 green LEDs, as shown in Figure 5a and Video S1, Supporting Information.The calculator was powered using the hybrid device, as shown in Figure 5b, and basic calculations were demonstrated in Video S2, Supporting Information.Figure 5c shows the powering of the hygrometer, and the demonstration was included in Video S3, Supporting Information.Powering the low-power electronics validated that the hybrid nanogenerator device exhibits a potential candidate for energy-harvesting and power sources for low-power electronics.
The prepared PB-TG device was used as the mechanical signal to the electric signal converting sensor.These sensors can be used for AR purposes.In general, AR needs accurate bending positions and holding capabilities.The finger motions were analyzed and displayed using the prepared SPA unit.The SPA consists of a TENG-based sensor, signal processing unit, and analyzer (computer software), as shown in the schematic Figure 5d.Arduino UNO was used as the signal processing unit.The unit will process the output signal from the sensor and give the input signal to the software for analysis.The software was constructed with the Arduino IDE, and processing IDE complies with the processing.The software will produce the bending states of the finger movement depending on the triggered sensor input.Using this type of sensing, holding, and placing, the object can be done in AR and VR.The Arduino analog signals for the different bending states and output display are shown in Figure 5e,f.The SPA demonstration was provided in Video S4 and S5, Supporting Information.

Conclusion
The multi-crystalline BCST material was systematically characterized with physiochemical and ferroelectric analysis.The hybrid composite film was prepared using the PDMS and BCST for the synergistic piezo and tribo effect.PENG, TENG, and hybrid nanogenerators were fabricated, and electrical characterizations were studied using the film.The integrated hybrid nanogenerator was double rectified, and the rectified voltage was obtained to be 326 V, which is higher than the piezoelectric and TENG.The hybrid device charged the commercial capacitor and powered the low electronics.SPA units were constructed to study the movement of the fingers.With the help of the software, real-time motions were observed.AR and VR systems can be built for healthcare, gaming, and education using this kind of system.

Experimental Section
Synthesis of Multi-Crystalline BCST Microparticles: The multi-crystalline BCST microparticles were prepared using the high-temperature solid-state reactions.The precursors like barium carbonate (BaCO 3 , 99%, Daejung chemicals), titanium dioxide (TiO 2 , 98%, Daejung chemicals), calcium carbonate (CaCO 3 , 99%, Daejung chemicals), and tin oxide (SnO 3 , 99%, Daejung chemicals) were used for the BCST preparation.Stoichiometrically weighed precursors were homogeneously mixed with the acetone for 1 h mechanically using a motor and pestle.The mixed precursor powder was calcinated at 1350 °C for 6 h.The calcinated powder was grained manually for 20 min.A few drops of polyvinyl alcohol were added to the final powder, and disk-shaped pellets were made using a mechanical load of 20 kN.The pellets were sintered at 1450 °C for 4 h to obtain the final BCST pellets.The sintered BCST pellet was grained to powder for further studies.
Preparation of Pure PDMS Film and PBC Film: The pure PDMS film was prepared using a silicone elastomer and curing agent with a ratio of 10:1.The solution was mixed manually and kept still for 30 min.The final solution was spin-coated at 100 rpm for 5 min on the aluminum foil.After drying, the pure PDMS film was obtained.BCST microparticles were mixed with the PDMS solution with 20 wt% filler loading concentration for the composite film preparation.The obtained composite solution was spin-coated at 100 rpm for 5 min on the aluminum foil.The prepared composite film was named PBC film.
Fabrication of Different Nanogenerators (PB-TG [TENG], PB-PG [PENG], and H-PB Hybrid Nanogenerator): The TENG (PB-TG) device was fabricated using the spin-coated PBC film used as the negative active layer backed with the aluminum electrode.Copper was used as the opposite layer and electrode.The PB-TG device was fabricated for the vertical contact separation mode with an arc-shaped configuration.The PENG (PB-PG) device was fabricated using the same hybrid composite film with the sputtered gold electrode on the top.The aluminum substrate acted as another electrode for the PB-PG device.The antistatic tape was placed over the electrodes to eliminate external influences.All the devices were fabricated with the dimension of 2 Â 3 cm 2 .An integrated hybrid device was constructed by coupling the PB-TG device and the PB-PG device with the help of an adhesive substrate.
Characterization: BCST and composite film crystalline analysis were evaluated at room temperature using X-ray diffraction (XRD) patterns (Rigaku; 40 kV, 40 mA, Cu-Kα radiation).Using a Raman spectrometer (model HR Evaluation; LabRAM), the symmetry and vibration were determined for all the samples.A field-emission scanning electron microscope (SEM) (TESCAN MIRA 3, Czech Republic) with an Oxford detector was used to examine the surface morphology and elemental composition.A precision material analyzer (Radiant Technologies, Inc. Precision 10 kV HVI-SC) was used to determine the ferroelectric polarization electric field loop as a function of frequency.The electrical properties such as the voltage, current, capacitor charging, stability, and instantaneous power density were determined with the electrometer (Keithley 6514) using a linear motor with an applied force (LinMot, Model No: HF01-37, USA).A voltage divider circuit was implied for measuring the voltage.

Figure 1 .
Figure 1.Structural, ferroelectric, and surface characterization.a) Rietveld-fitted X-ray diffraction (XRD) patterns of multi-crystalline 0.3Ba 0.7 Ca 0.3 TiO 3 -0.7BaSn0.12 Ti 0.88 O 3 (BCST), b) Raman spectrum of BCST, c) hysteresis loop and I-E curve of prepared BCST, d) hysteresis loop at the different applied electric field, e) remnant polarization at the different applied electric field, f ) coercive field at the different applied electric field, g) XRD analysis of pure polydimethylsiloxane (PDMS) and hybrid composite film, h) Raman spectrum of pure PDMS and hybrid composite film, and i) surface roughness mapping of hybrid composite film.

Figure 2 .
Figure 2. Electric characterization of triboelectric nanogenerator (TENG) (named as PB-TG).a) Layer-by-layer and device scheme of PB-TG, b,c) triboelectric output performance of PB-TG, d) stability test of PB-TG devices under applied continuous mechanical stress, e) load-matching analysis of PB-TG devices, and f ) output performance of PB-TG devices at different frequency.

Figure 3 .
Figure 3. Electric characterization of piezoelectric nanogenerator (PENG) (named as PB-PG).a) Layer-by-layer and device scheme of PB-PG, b,c) piezoelectric output performance of PB-TG, d) stability test of PB-PG devices under applied continuous mechanical stress, e) load-matching analysis of PB-PG devices, and f ) output performance of PB-PG devices at different frequency.

Figure 4 .
Figure 4. Electric characterization of integrated hybrid nanogenerator (named as H-PB).a) Layer-by-layer and device scheme of H-PB, b) rectified output performance of integrated hybrid H-PB device, c) capacitor charging of 1 μF commercial capacitor at the interval of 300 s, d) charge and energy stored in the capacitor (Q-E) plot, and e) charge and energy storage at different intervals.

Figure 5 .
Figure 5. Application of prepared energy harvester: a) lighting of 30 green LEDs, b) powering the commercial calculator, c) powering hygrometer, d) schematic of signal processing and analyzing unit, e) Arduino analog readings at different bending states, and f ) final output for indicating the different bending states of the finger.