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High-Performance Hybridized Composited-Based Piezoelectric and Triboelectric Nanogenerators Based on BaTiO3/PDMS Composite Film Modified with Ti0.8O2 Nanosheets and Silver Nanopowders Co-Fillers Saichon Sriphan, Thitirat Charoonsuk, Tosapol Maluangnont, and Naratip Vittayakorn ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00513 • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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ACS Applied Energy Materials

High-Performance Hybridized Composited-Based Piezoelectric and Triboelectric Nanogenerators Based on BaTiO3/PDMS Composite Film Modified with Ti0.8O2 Nanosheets and Silver Nanopowders Co-Fillers Saichon Sriphan,†,‡ Thitirat Charoonsuk,‡ Tosapol Maluangnont,§ and Naratip Vittayakorn*,‡,§,

†Faculty

of Science, Energy and Environment, King Mongkut’s University of Technology North Bangkok, Rayong Campus, Rayong 21120, Thailand ‡Advanced

Material Research Unit, Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand §Electroceramics

Research Laboratory, College of Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand Department

of Chemistry, Faculty of Science, King Mongkut's Institute of Technology Ladkrabang, Bangkok 10520, Thailand

Corresponding Author *E-mail: [email protected]

ABSTRACT: In order to commercialize the rapidly developing technology of energy harvesters, the following devices need to be developed further for enhancing output performance, flexibility, scalability, facile fabrication and cheaper price. The compositebased triboelectric nanogenerator (CTENG), which contains the above properties, is a promising technology that has attracted special interest for a decade. Focus has been placed on the hybrid concept between the composite-based piezoelectric nanogenerator (CPENG) and CTENG in order to enhance CTENG efficiency. This study presented a highperformance hybridized CPENG and CTENG device, which operated from the composite 1

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film of Ti0.8O2 nanosheets (Ti NSs)/silver nanoparticles (Ag NPs) co-doped BaTiO3 nanopowders (BT NPOs) inside the polydimethylsiloxane (PDMS) host. The 0.3 vol% of Ti NSs and 1.5 vol% of Ag NPs exhibited the optimum harvesting performance in all compositions, with an output voltage and current density reaching approximately 150 V and 0.32 µA/cm2, respectively. Their harvesting performance was approximately 60 and 32 times higher than that of the CPENG constructed from pure PDMS. In addition, practical demonstration of the proposed device was investigated. The hybridized CPENG and CTENG device could operate in a long-term cyclic operation, charge the capacitor for storing energy, and also drive LEDs to brighten. This work suggested facile device fabrication, and made a guideline to develop high-performance nanogenerators, which is crucial for device development and practical usage in the future. KEYWORDS: hybrid nanogenerators, triboelectric, piezoelectric, Ag nanoparticles, BaTiO3 nanopowders, TiO2 nanosheets

1. INTRODUCTION Nowadays, sustainable energy sources from the natural environment have been attractive in replacing fossil fuels, which are harmful and non-renewable, and lead to global warming.1-5 Among various kinds of energy harvesters, the mechanical type is a promising device that is capable of harvesting energy at all times.2-4 From an emerging energyharvesting technology, the piezoelectric nanogenerator (PENG) and triboelectric nanogenerator (TENG) have attracted special interest, due to their flexible structure, lowcost, simple design, high output performance and environmental friendliness.2-4 Under normal circumstances, the PENG has relatively low output power. The output voltage and current are typically in the ranges of sub voltages and nanoamperes. This limits specific 2


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applications, which require a high-performance nanogenerator. To overcome this obstacle, the PENG concept has focused on a hybrid with the TENG for enhancing nanogenerator output.2-3,6-12 It is common for the TENG to convert mechanical energies directly, e.g., from human movements, wind flow and ocean waves, to electrical signals in an alternating current (AC) formation. The working mechanism of the TENG is based on coupling triboelectrification and electrostatic induction by periodically rubbing together two tribomaterials and then separating them from each other during the application of an external pressing force.3,13 Regarding recent TENG technology, many strategies relating to the engineering of tribo-materials have been proposed to improve TENG efficiency, and develop it for novel applications.2-4 Study has focused mostly on composite-based tribo-material, which is prepared generally from the basis of organic polymers, for example, polyvinylidene fluoride (PVDF)11 and polydimethylsiloxane (PDMS).6-10,14,15 Among those materials, PDMS is one of the polymers studied most widely for self-sufficient power sources, due to its ability to gain electrons possessing significant properties of flexibility, transparency and durability.610,16

In order to improve electronic properties, PDMS is made homogeneously by coupling

with ceramic nanostructured materials, such as nanofibers,17 nanowires,18 nanoflakes,8 and nanopowders,6,7 to form a composite film. Owing to its simple, uncomplicated condition of growth and physical tunability, PDMS has become a suitable choice for engineering properties in achieving the high-performance composite-based piezoelectric nanogenerator (CPENG) and composite-based triboelectric nanogenerator (CTENG). Herein, a high-performance CTENG device, based on BaTiO3 nanopowders (BT NPOs) dispersed into PDMS elastomeric matrix was fabricated, in which its performance was compared simultaneously with the CPENG device. Reports of functional materials doped into composite-based tribo-material, as a new strategy for maintaining phase composition and engineering composite properties, have been scarce and rarely discussed. Therefore, Ti0.8O2 3



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nanosheets (Ti NSs) as a ceramic dispersant, and silver nanoparticles (Ag NPs) as conductive media, were co-doped into the BT NPOs/PDMS composite. A small amount of ceramic dispersant can disperse the main ceramic phase efficiently and produce more electric dipoles in composite film.19 The conductive phase capably provides conductive channels to any inorganic particle filler, resulting in more electrical charge generation.20-23 After mechanical agitation, both Ti NSs and Ag NPs were dispersed well in BT NPOs/PDMS. The Ti NSs/Ag NPs co-doped BT NPOs/PDMS composite film (3×4 cm2) was fabricated first for the CPENG device by easily sandwiching the composite layer in between two aluminum (Al) conductive tapes. During the application of external pressing force, the Ti NSs/Ag NPs codoped BT NPOs/PDMS CPENG produced a maximum electrical output voltage and current density of 40 V and 0.13 µA/cm2, respectively. After that, the CPENG device was constructed further to become the CTENG device (the hybridized CPENG and CTENG device was fabricated) by rubbing the Ti NSs/Ag NPs co-doped BT NPOs/PDMS composite film together with the Al plate. The result showed excellent electrical signals of 150 V and 0.32 µA/cm2, which were higher by over approximately 60 and 32 times, respectively, than those in the CPENG constructed from a pure PDMS device.

2. EXPERIMENTAL PROCEDURE 2.1. Materials. BT NPOs (99.95%, diameter 100 nm, Inframat Advanced Materials), silicone elastomer, silicone elastomer curing agent (Sylgard 184 silicone elastomer kit, Down Corning), thin Al plate (thickness 300 m), Al foil tape (3M 425, 3MTM Industrial Adhesives and Tapes), and phenolic epoxy glass (Epoxy G-11) were used. 2.2. Synthesis of Ti NSs. The Ti NSs were synthesized following the sequence of solid state synthesis-proton exchange-exfoliation (Figure S1). K0.8Zn0.4Ti1.6O4 was prepared 4


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first by calcining the stoichiometric mixture of K2CO3, ZnO and TiO2 twice at 900C for 20 h.24,25 Then, it was subjected to repeated proton exchange with 1 M HCl (overnight, renewed 3 times with fresh acid),26 giving H1.6Ti1.6O4·0.8H2O. The first 0.8H+ (per formula unit) is due to the exchange with 0.8K+. The second 0.8H+ was also incorporated into the solid, preserving charge neutrality due to the known Zn dissolution (0.8H+ for 0.4Zn2+). So, the total proton content is 1.6 per formula unit. The successful synthesis of these two precursors, including their characterizations and determination of the water content, has been reported elsewhere.27 Next, the H1.6Ti1.6O4·0.8H2O was shaken mechanically with tetrabutylammomium hydroxide [TBAOH, (C4H9)4NOH, 1 M in water, Sigma-Aldrich, Co., Ltd. USA] at 180 rpm for 14 days. The solid-to-solution ratio was fixed at 0.4 g to 100 mL, with the mole ratio of H+ (in the solid) to TBA+ (in the solution) equal to 1.26 The resulting white Ti NSs colloid, i.e., Ti0.8O2 nanosheets was obtained finally. Restacking was performed by adding 100 mL of 2 M KOH into 100 mL of the Ti NSs colloid. The precipitate was washed with deionized water until it was free from excess base, followed by drying overnight at RT. 2.3. Synthesis of Ag NPs. Regarding Ag NPs synthesis, the reagents were silver nitrate (AgNO3, Sigma-Aldrich, Co., Ltd. USA), sodium borohydride (NaBH4, SigmaAldrich, Co., Ltd. USA) and poly(4-styrenesulfonic acid-co-maleic acid) or PSSA-MA (sodium salt, molecular weight D 20,000 g/mol; Sigma-Aldrich, Co., Ltd. USA). All of the chemicals used were analytical grade and used without further purification. All of the solutions were prepared with double-distilled water (resistance >18 M/cm). Chemical reduction by reducing agents was selected in this work for synthesizing Ag NPs. NaBH4 was used for reducing silver ions (Ag+) in aqueous solution. PSSA-MA was utilized as a capping agent for stabilizing dispersive NPs during the course of preparing metal ones, and avoiding 5



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their agglomeration. Schematic illustration of Ag NPs synthesis using chemical reduction is shown in Figure S2a. The preparation steps can be summarized as follows: a 1mM solution of AgNO3 was mixed with an equal volume of 1 mM PSSA-MA. Then, 100 ml of freshly prepared 10 mM NaBH4 was added quickly to the Ag/PSSA-MA mixed solution under vigorous stirring. After 24 hours, the color of the solutions turned from colorless to yellow, which confirmed the formation of spherical Ag NPs. The Ag NPs colloids were then kept in closed containers at 4C. 2.4. Fabrications of BT NPOs/PDMS and Ti NSs/Ag NPs co-doped BT NPOs/PDMS Composites. In order to fabricate the BT NPOs/PDMS composite film, 5 vol% BT NPOs were loaded into a vicious PDMS, which was prepared by mixing silicone elastomer and silicone elastomer curing agent in a weight ratio of 10:1 (Figure 1a). The overall volume of mixed composite was set at 5 ml. The mixture of BT NPOs and PDMS was poured into a square mold sized 342 cm3 (Figure 1b). The mixture for curing was loaded in an electrical furnace at 50C for 3 h, and also exposed to air for 6 h. The Ti NSs/Ag NPs codoped BT NPOs/PDMS composite film was prepared in the same manner, with the Ti NSs doped into the mixture of BT NPOs and PDMS at various compositions between 0 and 0.5 vol% before curing. The 0 to 1.9 vol% Ag NPs also were poured into that mixture for additional doping. The mixture of Ti NSs/Ag NPs co-doped BT NPOs/PDMS was cured in the furnace at 50C for 3 h and exposed to air for 6 h. Figure 1c shows the composite sample with the same size as the square mold, and a thickness of 1 mm, obtained by peeling the film from the mold after curing. This work found that the fabricated composite film was very flexible, and could be bent up to 60 from a vertical axis (Figure 1d). This demonstrated the effectiveness of the proposed composite film as being a flexible nanogenerator.

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2.5. Fabrications of the CPENG and CTENG Devices. The CPENG can be fabricated by following 3 processes: (1) a composite film sandwiched between two Al tapes, (2) copper wires attached to make electrodes, and (3) a package device with thin-film Kapton tape to protect it from external environmental effects (Figure 2a,b). In order to prepare the CTENG, the bottom side of a composite film was attached with Al tape, and then doublesided tape, to copper wire on an Epoxy G-11 plate, which was used as the bottom substrate. The effect of using this plate was its flexural and compression strengths, which are much higher than those of the commercial Acrylic plate of the same size (ASTM D790 standard). Four metal springs with long screws and locknuts were used to support the top substrate, which consisted of an Al plate installed with copper wire, double-sided tape and an Epoxy G11 plate. Structure of the CTENG device is shown in Figure 5a,b. 2.6. Characterizations. The Ti NSs colloids were characterized by the following procedures. The ultraviolet-visible (UV-Vis) spectrum was recorded after dilution (0.2 mL of the original colloid, diluted to 100 mL), using a T90+ UV/Vis spectrometer (PG Instruments) from 200-500 nm. Deionized water was used for baseline subtraction. Dynamic light scattering (DLS) was performed on the as-made colloid (without dilution) using a DelsaTM Nano Particle Size Analyzer (Beckman Coulter). The particle size of Ti NSs, i.e., the hydrodynamic radius (RH), was calculated using the Stokes-Einstein equation, assuming that the NSs are rigid spheres.28 Transmission electron microscopy (TEM) images of deposited Ti NSs were acquired from a JEOL JEM-2010 TEM. In addition, the Ag NPs colloids also were characterized using a UV−Vis spectrophotometer [T90+ UV/Vis spectrometer (PG Instruments)] and transmission electron microscope (JEOL JEM-2010 TEM). The waveforms of output voltage and current, which have the same meaning as open-circuit voltage and short-circuit current, respectively, were recorded via a digital multimeter (DM3058E, Rigol). For the mechanical test, a home-made automatic pressing machine installed with air pump 7



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(exhaust pressure of 6 bar) was used. This was operated by the air flow-based concept (Pneumatic system). The compressive force was applied in a vertical direction with the exerted force of 247 N and the operating frequency of 0.5 Hz.

3. RESULTS AND DISCUSSION 3.1. Property and Morphological Characterizations of Ti NSs and Ag NPs. As described previously, the Ti NSs were prepared in aqueous colloid formation after the exfoliation of H1.6Ti1.6O4·0.8H2O with TBAOH. The Ti/Zn atomic ratio of this protonic layered material, as determined from XRF, is 96.6. This number is substantially larger than 3.61 experimentally found for the starting K0.8Zn0.4Ti1.6O4, which is in reasonable agreement with the nominal value of 4 (=1.6/0.4). So, the subsequent formation of nanosheets containing mostly Ti and O with very small amount of Zn can be safely assumed. The white colloid exhibited a Tyndall effect when the laser light shined brightly (inset of Figure S3a), suggesting the presence of dispersed objects (i.e., NSs).26 The colloid showed characteristic absorption at 262 nm (Figure S3a), which is in good agreement with the literature.27,29 This result indicates the infinite separation of the layered crystals, H1.6Ti1.6O4·0.8H2O, into elementary layers. Figure S3b shows the intensity-averaged particle size of the objects in the colloid of 268 nm, as determined from the DLS result. While DLS assumes that dispersed objects are spherical and rigid, which clearly is not the case for flexible and anisotropic NSs, it provides a fast and convenient size estimation. The TEM image of Ti NSs in Figure 1c clearly shows that the objects, as deposited from the colloid, are two-dimensional (2D). The long dimension of ~200 nm was found in the majority of NSs, although there is a wide distribution of lengths and widths. Considering that these two techniques, i.e., DLS and TEM,

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were used to measure the size of NSs in different physical states (dispersed vs deposited), the size obtained of 200-300 nm is in reasonable agreement. The local structure of the Ti NSs was indirectly studied by Raman spectroscopy of the Ti NSs restacked with K+ ions, in relation to the starting H1.6Ti1.6O4·0.8H2O, Figure S3c. The spectrum of H1.6Ti1.6O4·0.8H2O shows the peak at 178, 265, 386, 449, 560, 729 and 914 cm-1, consistent with lepidocrocite protonic nanotubes.30 These peaks are due to different Ti-O vibrational modes differing in the coordination around Ti, and also the Ti-O bond lengths. Upon restacking, the majority of peaks were kept at the same positions, although with a change to the relative intensity. The peaks at 513 and 638 cm-1 were most likely shifted from those at 560 and 729 cm-1 respectively. The peak shifting of the same magnitude was previously reported in the restacked Ti NSs (by NaOH) compared to the corresponding protonic titanate.30 The difference of peak positions in our work vs that by Gao et al. might be related to the restacking agent (KOH vs NaOH) and/or the composition of the titanate (H1.6Ti1.6O4·0.8H2O vs H0.7Ti1.825O4·H2O). Yet, one can deduce that the lepidocrocite structure of the parent solid is preserved in the resulting Ti NSs, and that the successful synthesis of high-quality Ti NSs has been achieved. Unfortunately, it is not possible to obtain the Raman spectrum of Ti NSs in the composite (later) due to its small amount. The absorption spectrum of the Ag NPs colloids is shown in Figure S2b. The spectrum exhibits a plasmon absorption band at ~ 400 nm, which is the characteristic of Ag NPs. In consequence, the TEM results showed that Ag NPs exhibited mono-dispersed spherical particles with an average particle size of about 7.3  2.6 nm (n = 150) and a narrow size distribution, as shown in Figure S2c,d, respectively. 3.2. Charge Generation Mechanism of the CPENG Device. Figure 2a shows the device structure of the CPENG, which mainly consists of composite film as the charge 9



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generation layer. The real device for utilizing the electrical test is shown in Figure 2b. The working mechanism of the CPENG device is shown in Figure 2c. This mechanism is based mainly on the piezoelectric property of BT NPOs and Ti NSs embedded inside PDMS, under the application of external forces.20 There is no electrical output transferred to the load in the initial stage (stage I) because the electric dipoles generated from the BT NPOs and Ti NSs inside the composite are balanced. The electrical charges in piezoelectric composite film is cancelled out, leaving no net charge on the film surface. However, the piezoelectric charges are generated after the application of an external force (stage II). Electric dipoles are unbalanced overall during material deformation, owing to their stress-induced orientation inside the composite sample.20,21,23 In this case, a piezoelectric potential gradient throughout the composite is created. The net positive and negative charges appear on the opposite film surfaces, leading to a flow of piezoelectric charges (or output current) from the composite film. Zero output signal is observed at the equilibrium of deformation (stage III). After removal of an external force, the compressive stress is released (stage IV). The generated charges on the film surfaces flow back to maintain the material equipoise. The production of output current is contrary. After the complete of charges transferring process, the highest value of electrical signal is observable until the deformation reaches to the original state (stage V). From this case, the AC electrical signals are generated periodically, with a cyclic application of an external force. 3.3. Electrical Output Performance of the CPENG Device. Figure 3 shows the electrical output performance of the CPENG device at various Ti NSs concentrations. External pressing force was applied stably at the frequency of 0.5 Hz, with an environmental humidity of 50%. The output voltage and current were measured under the same conditions. It has been seen that the output voltage and current density of BT 10


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NPOs/PDMS (green solid line) and Ti NSs-doped BT NPOs/PDMS (blue solid line) were increased remarkably when compared with those of pure PDMS (black solid line), as shown in Figure 3a,b. With regard to the BT NPOs/PDMS CPENG device (5 vol% of BT NPOs embedded inside the PDMS), the averaged output voltage and current density reached 8 V and 0.02 µA/cm2, respectively. These values were about 3.2 and 1.8 times higher, respectively, than those when the CPENG operated from a pure PDMS (Figure 3c,d). By doping 0.1 to 0.3 vol% of Ti NSs into BT NPOs/PDMS composite, the averaged output voltage enhanced further to approximately 17 V. The averaged output current density increased to approximately 0.03 µA/cm2. Enhancement was about 7 and 3 times higher for the averaged output voltage and current density, respectively, when compared with that in the pure PDMS device (Figure 3c,d). The electrical output performance tended to degrade with Ti NSs filling increasing to 0.5 vol%. This might be due to the influence of over-doping, which could cause a higher level of BT NPOs and Ti NSs agglomerations. The BT NPOs dispersion inside the PDMS was degenerated, resulting in the decrease of generated electrical output.6,19 This work thus concluded that 0.3 vol% of Ti NSs doped into the BT NPOs/PDMS composite was the optimum condition. Based on analysis from the authors’ previous work,31 the BT NPOs tended to have a tetragonal structure. Hence, the increase of the output voltage and current density might come from the effect of piezoelectric BT NPOs inside the PDMS. With doping of sheet structured Ti NSs27 to the BT NPOs/PDMS composite, the electrical output performance of the CPENG was improved further. This was due to additional piezoelectric charges, which were generated from the 2D Ti NSs phase. The results from this work confirmed the role of piezoelectric effect on CPENG performance, which is crucial for nanogenerator development. In order to increase the amount of electrical charge output further, metal nanomaterials were added into the composite film.21,22 Huan et al.21 demonstrated that, by 11



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adding Ag NPs to (K0.5Na0.5)NbO3/PDMS composite, a substantial increase of electrical output was observable. Therefore, this work selected Ag NPs to improve conductivity of the composite charge in the BT NPOs/PDMS composite film by doping them together with 0.3 vol% Ti NSs as the optimum condition. The output voltage and current density increased significantly by continuously doping Ag NPs from 0.3 to 1.5 vol%, as shown in Figure 4a,b. It was worth noting that the electrical output tended to reduce slightly to 1.9 vol% after doping Ag NPs. Too much metal nanomaterial doping was found to deteriorate the electrical performance of the CPENG device.21,22 Even though the conductivity charge of the composite film improved, some metal nanomaterials, which attached to the BT NPOs, could affect individual polarization of the insulating materials, i.e., BT NPOs and Ti NSs inside PDMS. In this case, the quantity of piezoelectric charge might be reduced, causing a device degradation. The highest output voltage of 40 V and the highest output current density of 0.13 µA/cm2 were found at 1.5 vol% of Ag NPs. When comparing with the pure PDMS device, both output voltage and current density of the Ti NSs/Ag NPs co-doped BT NPOs/PDMS CPENG were about 16 and 13 times higher, respectively (Figure 4c,d). After doping, the effect of Ti NSs and Ag NPs co-doped into BT NPOs/PDMS composite could boost the output performance of the CPENG device effectively, as verified from electrical output enhancement. 3.4. Charge Generation Mechanism of the CTENG Device. It is well known that applying the TENG concept to engineered composite film for making the hybrid between the PENG and TENG could improve nanogenerator performance significantly.6-11,18,22,32 Hence, this work selected its composite film with the optimum conditions of 0.3 vol% Ti NSs and 1.5 vol% Ag NPs co-doped into BT NPOs/PDMS to fabricate the CTENG device by rubbing the composite film together with the Al plate (Figure 5). This work chose the Al plate as rubbing material because Al is far away from PDMS in the triboelectric series.33,34 The 12


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PDMS composite has a tendency to collect electrons (negative charge), while the Al tends to lose them (positive charge), thus inducing electric field distribution during rubbing. Structure of the CTENG device is shown in Figure 5a. The Ti NSs and Ag NPs co-doped BT NPOs/PDMS composite film were sandwiched between the substrate (Epoxy G-11) attached to the Al conductors. This work maintained the gap of around 1 mm between the composite film and Al plate. A photograph of the real CTENG device is shown in Figure 5b. The working mechanism of the CTENG device (Figure 5c) cannot produce triboelectric charges in the original state (stage I) because it lacks friction between the two tribo-materials, i.e., the composite film and Al plate. However, when an external pressing force is completely applied to the TENG (stage II), the surface of two tribo-materials touch and rub against each other. The triboelectrification process begins to perform at this stage. In addition, the Ti NSs, Ag NPs and BT NPOs in the PDMS matrix also undergo compressive force. The piezoelectric effect takes place (explained previously in section 3.2), and generates the additional surface charges supporting to the triboelectric effect. In this case, the dual phenomena of piezoelectric and triboelectric effects can lead to more increase of the final electrical output of CTENG device. Owing to coupling of electrification contact and electrostatic induction,3,13,33 the composite PDMS, as the stronger triboelectric affinity for a negative charge, steals the electrons from the Al plate surface. Top Al plate becomes positively charged, while the composite film surface contains negatively charged. The amount of stolen electrons is proportional directly to the generated charges from the hybrid piezoelectric and triboelectric effects. Electric polarity is generated after separation of the two tribo-materials (stage III). The production of electric polarity obeys electrostatic induction, in which the composite film induces its opposite generated triboelectric charges from the top Al plate to the bottom Al electrode.3,13,33 Electrons were driven to the top Al plate, resulting the generation of a positive output current. During release, the induced charges transfer 13



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numerously through the external circuit. This leads to the maximum peak in the electrical output. However, there is zero current (no charge transfer) at stage IV, due to charge neutrality if the stretching step releases to equilibrium. The induced charges flow back to their original position by immediately pressing perpendicular to the CTENG (stage V). The piezo- and tribo-induced charges are recovered, leading to a change of electric potential.3,13,33 The current flows to the contrary. Therefore, the AC electrical output for both voltage and current of the CTENG device is observed when the mechanical pressing is applied consecutively. 3.5. Electrical Output Performance of the CTENG Device. Figure 5d shows the electrical output performance of the CTENG device. Regarding the measurement, the periodic pressing force applied on the CTENG has the same conditions for testing as those on the CPENG. The output voltage and current density reached approximately 150 V (Video S1) and 0.32 µA/cm2, respectively, with about 60 and 32 times increase, respectively, when compared with the CPENG, which was constructed from a pure PDMS. The fabricated CTENG has an output voltage and current density of about 3.8 and 2.5 higher, respectively, than the optimum CPENG in order of magnitude. From a previous section of this work, the significant enhancement of nanogenerator performance was believed to arise from the hybrid PENG and TENG concept. In order to investigate the hybrid mechanism of the proposed device, the contributions of output voltage and current density for three different modes: piezoelectric, triboelectric and hybrid, was considered as shown in Figure 5e. The individual piezoelectric output (PO), triboelectric output (TO) and hybrid output (HO) were measured. In this case, the individual piezoelectric effect was achieved by that the Al plate was moved from the top substrate of the CTENG structure (Figure 5a) to become attached, without leaving a gap between the composite film. This was the same structure as the fabricated CPENG device. After the pressing test, it was found that the recorded output voltage and 14


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current density had the same range as the electrical output obtained from the CPENG device, which were reported previously (40 V and 0.13 µA/cm2) (Figure 4). The individual triboelectric effect was investigated by friction between pure PDMS and Al plate. The observed output voltage and current density of approximately 50 V and 0.17 µA/cm2 were in the same trend as the results reported from Chun et al.10 and Kim et al. 35 works. The combination of triboelectric and piezoelectric effects could greatly enhance the electrical performance of the nanogenerator (boosted from 40 to 150 V, and 0.13 to 0.32 µA/cm2) as seen in Figure 5e. The additional charges of the engineered composite film from the piezoelectric effect played an important role in greatly improving the TENG performance. Hence, the hybrid concept of CPENG and CTENG devices opened up new avenues for a high-performance nanogenerator development. 3.6. Summation for the Device Performance of the CPENG and CTENG. The device performance, which was presented in the formation of output voltage and current density dependences for each CPENG and CTENG device used in this work, is summarized in Figure S4a. To simplify the name and category of the devices, pure PDMS, BT NPOs/PDMS, Ti NSs doped BT NPOs/PDMS and Ti NSs/Ag NPs co-doped BT NPOs/PDMS, were called device 1, 2, 3 and 4, respectively. The electrical output from these devices was recorded, according to the piezoelectric mechanism. The CTENG was called device 5, which complied with the triboelectric effect. Device 1 was constructed from pure PDMS and device 2 used 5 vol% BT NPOs embedded in PDMS matrix. Device 3 was doped Ti NSs in 0.3 vol% ratio into BT NPOs/PDMS composite film from optimum conditions. The 0.3 vol% Ti NSs and 1.5 vol% Ag NPs were co-doped into BT NPOs/PDMS composite for device 4 (Figure 3,4). High-performance CTENG (device 5) was achieved by applying device 4 in a composite film from rubbing together with the Al plate (Figure 5). The electrical output 15



Page 16 of 36

data from device 1 to 3 tended to aggregate, meaning that the output performance of these devices had very little enhancement, as shown in Figure S4a. It was interesting that the increasing trend of output voltage and current density showed a very high slope in device 4. This was good proof that co-doping Ti NSs and Ag NPs phases into BT NPOs/PDMS composite could improve nanogenerator outputs efficiently. After making device 5, the output voltage and current density had considerably more enhancement. The substantial increase of electrical output of this device was due to the piezo-induced triboelectrification from the hybrid concept, which has been verified by many works.6-11,22,35 Table 1 shows the comparison of the electrical output performance of hybrid piezoelectric and triboelectric nanogenerators between this work and others from the literatures.6-11 The CTENG in this work had an averaged maximum voltage higher than that previously reported for the conductor-to-dielectric contact-mode in CTENGs. In addition, the averaged maximum current density obtained was in the same trend as that in the modified CTENG using Al as rubbing material.7,10 However, the output current of the device obtained in this work was still lower than that in some CTENG reports, which used a poling aid and high-cost complex composite structure.6,8,9,11 In comparison, it can be concluded that the CTENG device in this work contained co-filling of Ti NSs and Ag NPs, and is able to improve the electrical output of a conventional BT NPOs/PDMS device in a cheaper structure, without the addition of a poling aid or composite modification. Based on a theoretical model used to explain the conductor-to-dielectric contact-mode of TENG,13 the relationship of voltage V - transferred charge Q - separation distance x (V-Qx) is given by

V 

  x t  Q d ,   x t    S 0   r 0 

(1)

16


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where S is the device area,  0 is the vacuum permittivity,  r is the relative permittivity of the composite, d is the active layer thickness and  is the surface charge density (SCD). There is no charge transfer (Q = 0) through the circuit in the open-circuit condition. Thus, the opencircuit voltage VOC , which has the same meaning as the output voltage, can be written as

VOC 

 x t  , 0

(2)

V is equal to zero in the short-circuit condition. The transferred charges are therefore QSC 

S x  t  ,  d /  r   x t 

(3)

From this condition, the short-circuit current I SC , which is the output current presented in this work, was obtained as I SC 

S  d /  r  d d  S x  t   dx QSC   .  2 dt dt   d /  r   x  t    d /  r   x  t   dt  

(4)

It was observed from eq 2,4 that the SCD is a main factor affecting the electrical output of TENG. The amount of transfer electric charge (TEC) during pressing also was investigated. The TEC results for different devices are shown in Figure S4b. This factor was achieved by integrating the AC output current waveforms recorded from device 1 to 5. In this case, only the positive half cycle was integrated. The area obtained in each half cycle was averaged to find the averaged TEC data.32 The maximum TEC (0.037 µC) was found by doping Ti NSs and Ag NPs together into BT NPOs/PDMS composite for the CPENG device (device 4). The SCD was investigated, as a reference point for considering performance of the nanogenerators in µC/m2,36 and the TEC calculation was divided by the size of the composite film (34 cm2). Device 4 was found to have the highest SCD of 31 µC/m2. This value was large, when compared with devices 1 to 3. The SCD value reached 72 µC/m2 in device 5, 17



Page 18 of 36

which was approximately 29.3 and 2.3 times higher than that in device 1 and 4, respectively. Apart from the hybrid concept, the effectiveness of doping can prove performance enhancement of the device when considered in terms of d /  r in eq 1-4.12,37 With the constant value of the composite film thickness of the CTENG in this work, the  r factor of NPOs/PDMS composite can vary from various Ti NSs and Ag NPs dopants. Hence, the improvement of SCD from device 5 can be achieved by the optimal addition of Ti NSs and Ag NPs. 3.7. Device Performance Test for the CTENG Device. Figure 6 depicts the device performance of the CTENG in this work. Before using real-life applications, i.e., those for driving electrical loads such as light emitting diodes (LEDs), diodes, solar cells, etc., the CTENG device needs to convert its AC signal for both output voltage and current to a direct current (DC) signal. The diode bridge rectifier is a well-known system used to convert an AC to a DC signal (Figure 6a).38,39 The converter comprises four diodes (D1 to D4) connected together in a bridge formation. The effect of diodes enables one-way charge flow (DC signal production), even though the signal amplitude is reduced slightly.38 DC output of the CTENG device was used to drive various load resistances ranging from 1 k to 1 G. The obtained output voltage and current density are shown in Figure 6b. The output voltage tended to increase when load resistance increased, while the output current decreased. The instantaneous power density of the CTENG as a function of external load resistance is shown in Figure 6c. This factor can be calculated easily by multiplying the current load by the voltage load at various resistances. The highest output power was 320 W/cm2 under a load resistance of 80 M. This corresponded to the internal resistance of the CTENG device by following the maximum power transfer theory.40 High internal resistance was mainly due to the influence of insulating inorganic materials, which were BT NPOs and Ti NSs, co-filled in 18


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PDMS. The doping of Ag NPs into PDMS composite near the percolation threshold could additionally serve the increase of dielectric permittivity of composite.40 Hence, the total resistance of the present composite film would augment and showed the highest delivered power at very high resistance after being the CTENG device.34,41 The rectified output voltage within 60 s is shown by the inset in Figure 6c. The averaged DC output voltage of approximately 100 V was consistent with the general reduction of electrical signal amplitude after being rectified.7,9 Figure 6d shows charging behavior of the CTENG device in order to investigate the energy storage application. While charging three commercial capacitors, the CTENG in this work could store 20 V of output voltage by charging 0.22 µF within 15 s, whereas the other capacitors were starting to charge. The output stability of the CTENG is shown in Figure 6e. The output voltage was recorded continuously within 1,800 s (900 cycles). It was rather stable without device degradation, indicating good performance of the CTENG device in this work. Furthermore, the CTENG device could drive nine commercial LEDs effectively during the pressing test, as shown in Video S2 (Figure 6f). This demonstrated a practical usage of the CTENG device in this work, which was capable of being utilized as an efficiently promising power supply for small electronic devices.

4. CONCLUSION High-performance hybrid CPENG and CTENG, using co-doping Ti NSs and Ag NPs into BT NPOs/PDMS composite was demonstrated as an active material. The results indicated that doping Ti NSs into the PDMS host, in which 0.3 vol% was the optimum condition, improved electrical output of the BT NPOs/PDMS CPENG. The averaged output voltage and current density were approximately 17 V and 0.03 µA/cm2, respectively. This device could enhance performance further by optimizing the Ag NPs dopant concentration. 19



Page 20 of 36

This work found that 0.3 vol% Ti NSs and 1.5 vol% Ag NPs co-doping into BT NPOs/PDMS showed the highest electrical outputs for the CPENG device, which were 40 V and 0.13 µA/cm2 of output voltage and output current density, respectively. With regard to the hybrid concept, the optimum composite film used for the CPENG was constructed as the CTENG device. The Ti NSs/Ag NPs co-doped BT NPOs/PDMS composite film was sandwiched between the substrate (Epoxy G-11) attached to Al conductors by maintaining the frictional gap of around 1 mm. The recorded output voltage and current density from the CTENG reached approximately 150 V and 0.32 µA/cm2, respectively, which was 60 and 32 times higher, respectively, when compared with the CPENG, which was constructed from a pure PDMS, and 3.8 and 2.5 times higher, respectively, than the optimum CPENG. The highest output power could reach up to approximately 320 W/cm2 under a load resistance of 80 M. The CTENG device could generate a stable electrical output in a long-term cyclic operation. It could charge the capacitor to store the voltage at around 20 V within 15 s, and also drive many LEDs to brightness. This work proved the efficient concept of developing high-performance CTENGs by using the hybrid formation, and suggested the essential strategy for engineering composite film by doping multifunctional nanostructured materials.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Schematic diagram for the synthesis of Ti NSs and Ag NPs and figures with additional characterization data of materials and CTENG device (PDF) Video S1 showing output generation of the CTENG (MOV) 20


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Video S2 showing LEDs lighting up by the CTENG (MOV)

AUTHOR INFORMATION Corresponding Author *E-mail [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors discussed the results and commented on the manuscript. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS This work was supported financially by Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang under Grant no. 2561-01-05-77. The work of T. Charoonsuk was supported by KMITL Grant No KREF146201. And T. Maluangnont acknowledges the financial support from the National Research Council of Thailand.

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Table caption

Table 1. Electrical output Performance of Hybrid Piezoelectric and Triboelectric Nanogenerators Reported from the Literature6-11

Table

Table 1. Electrical output Performance of Hybrid Piezoelectric and Triboelectric Nanogenerators Reported from the Literature6-11 Ref.

Device

Working

No.

Structure

Mechanism

(6)

Cantilever

Triboelectric,

Resonator

Piezoelectric

Rectangle

Triboelectric,

(7)

Composite Material

BT NPOs; PDMS

Rubbing

Poling

Averaged Maximum

Material

Condition

Voltage/Current Density

Copper

120 kV/cm

13.5 V/ 17.2 A/cm2

for 12 h BT NPOs; PDMS

Al

None

60 V/ 0.44 A/cm2

Triboelectric,

ZnO Nanoflakes; 3D

Gold

None

62.5 V/ 26.8 A/cm2

Piezoelectric

Graphene; PDMS

Triboelectric,

MWCNT; PDMS

P(VDF-TrFE)

None

12.5 V/ Not Available

Al

None

150 V/ 0.62 A/cm2

Piezoelectric (8) (9)

Tandem D-Shaped

Piezoelectric (10) (11)

Rectangle Rectangle

Nanofibers; Ag

Triboelectric,

Au NPs; Porous

Piezoelectric

PDMS

Triboelectric,

Ti NSs; PVDF

Copper

None

52.8 V/ 5.7 A/cm2

Triboelectric,

Ti NSs; Ag NPs; BT

Al

None

150 V/0.32 A/cm2

Piezoelectric

NPOs; PDMS

Piezoelectric Our Work

Tandem

27



Page 28 of 36

Figure legends

Figure 1. (a-c) Fabrication steps of the Ti NSs/Ag NPs co-doped BT NPOs/PDMS composite film and (d) flexible demonstration of fabricated composite film. Figure 2. (a) Structural diagram, (b) real photograph and (c) proposed working principle for the CPENG device. Figure 3. (a) Output voltage and (b) output current density of the CPENG device with different Ti NS contents. Trends of (c) positive output voltage and (d) positive output current density for the CPENG device at various Ti NS contents. Blue-dotted symbol and red-solid line represent the experimental data and non-linear curve fitting line, respectively. Factor of fitting correlation coefficient (R) is calculated for determining the quality of non-linear curve fitting to the recorded data. The non-linear curve fitting line is drawn for guiding the eye in observing tendency of the filling volume ratio. Figure 4. (a) Output voltage and (b) output current density of the CPENG device with different Ag NP contents. Trends of (c) positive output voltage and (d) positive output current density for the CPENG device at various Ag NP contents. Blue-dotted symbol and red-solid line represent the experimental data and non-linear curve fitting line, respectively. Factor of fitting correlation coefficient (R) is calculated for determining the quality of non-linear curve fitting to the recorded data. The non-linear curve fitting line is drawn for guiding the eye in observing tendency of the filling volume ratio. Figure 5. (a) Structural diagram, (b) real photograph and (c) proposed working principle of the CTENG device. (d) Output voltage and output current density for the CTENG. (e) Extracted electrical signals for investigating the piezoelectric output (PO), triboelectric output (TO) and hybrid output (HO) of the CTENG device.

28


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Figure 6. (a) Diode bridge rectifier for converting AC to DC outputs for the CTENG device. (b) Output voltage and output current density dependences on load resistance for the CTENG device. (c) Calculated instantaneous power corresponding to the resistance load. Inset shows the DC output voltage recorded within 60 s. (d) Charging characteristic of the CTENG device at various capacitance loads. (e) Durability test of the CTENG device recorded for 1,800 s. (f) Photograph of the CTENG driving nine blue LEDs.

29



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Figures

Figure 1. (a-c) Fabrication steps of the Ti NSs/Ag NPs co-doped BT NPOs/PDMS composite film and (d) flexible demonstration of fabricated composite film.

30


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Figure 2. (a) Structural diagram, (b) real photograph and (c) proposed working principle for the CPENG device.

31



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Figure 3. (a) Output voltage and (b) output current density of the CPENG device with different Ti NS contents. Trends of (c) positive output voltage and (d) positive output current density for the CPENG device at various Ti NS contents. Blue-dotted symbol and red-solid line represent the experimental data and non-linear curve fitting line, respectively. Factor of fitting correlation coefficient (R) is calculated for determining the quality of non-linear curve fitting to the recorded data. The non-linear curve fitting line is drawn for guiding the eye in observing tendency of the filling volume ratio.

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Figure 4. (a) Output voltage and (b) output current density of the CPENG device with different Ag NP contents. Trends of (c) positive output voltage and (d) positive output current density for the CPENG device at various Ag NP contents. Blue-dotted symbol and red-solid line represent the experimental data and non-linear curve fitting line, respectively. Factor of fitting correlation coefficient (R) is calculated for determining the quality of non-linear curve fitting to the recorded data. The non-linear curve fitting line is drawn for guiding the eye in observing tendency of the filling volume ratio.

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Figure 5. (a) Structural diagram, (b) real photograph and (c) proposed working principle of the CTENG device. (d) Output voltage and output current density for the CTENG. (e) Extracted electrical signals for investigating the piezoelectric output (PO), triboelectric output (TO) and hybrid output (HO) of the CTENG device.

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Figure 6. (a) Diode bridge rectifier for converting AC to DC outputs for the CTENG device. (b) Output voltage and output current density dependences on load resistance for the CTENG device. (c) Calculated instantaneous power corresponding to the resistance load. Inset shows the DC output voltage recorded within 60 s. (d) Charging characteristic of the CTENG device at various capacitance loads. (e) Durability test of the CTENG device recorded for 1,800 s. (f) Photograph of the CTENG driving nine blue LEDs.

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Highlights 

Performance of the CPENG is improved by co-doping Ti NSs and Ag NPs into BT NPOs/PDMS composite film.



The high-field poling-aided process is not necessary in improving performance of the proposed device.



Using the hybrid concept of CPENG and CTENG can enhance the nanogenerator performance significantly.



The present CTENG device demonstrates a reliability for being a practical power source.

Graphical Abstract

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High-Performance Hybridized Composited-Based ... - ACS Publications

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